there appears to have been some sort of reaction with glue squeeze-out that has caused a sort of 'ebonzing' stain from the face of the clamp, through the glue puddle and into the wood. I hang my head in shame - I should have packed top and bottom and I promise I will NEVER take that short-cut again.
This is a common reaction between steel and PVA glues, causing a kind of blue-black staining. You're not the first to experience this (been there myself!) and I promise you you won't be the last so don't beat yourself up too much.
I have sanded most of the staining out but in a couple of patches it seems to go on forever and I am getting worried about planing or sanding deeper (bearing in mind that I don't have a planer, being a hobbyist, so must do it by hand and the maple has some pretty, but gnarly grain that tears out as soon as look at it).
Sanding is a perfectly acceptable way of doing this, although far from my favourite, it just tends to be slowest. Staining often seems very deep when you're trying to sand it out because sanding is really quite slow. Individual plane shavings from a smoother can be the equivalent of about five minutes of sanding (!) so you get down through the wood much faster, but also in a very controlled way as you're taking off perhaps a few thousandths of an inch (less than 0.1mm) at a time.
So, here's the question: is there any trick to gently de-ebonize these patches or will I have to just keep planing my otherwise finished table top?
IME the best way to deal with this on a flat surface is to plane it out. But if you have some gnarly grain to deal with this is only viable if you have your hand plane set up well enough that you won't tear out the surface1. An alternative is scraping, which is also much more efficient than sanding.
Bleaching it out
Bleaching may work to get this out but you want to be careful which bleach you buy as there are three used in woodworking.
Oxalic acid is the one to get. Oxalic is the go-to for metallic stains and for removing dark water staining (which are usually from dissolved iron in the water reacting with the wood). It can be bought in readymade solutions, or as a powder and made up into a liquid with water (usually hot water)2. See notes on using oxalic acid for further details.
I suggest this is the first thing to try, because it may remove the staining completely. And if it doesn't work you then know you have no choice but to continue to remove material to try to get beneath the stain.
Avoiding the problem in future
As you've already determined, prevention is better than cure for something like this and one way to help avoid the possibility of this happening is to glue pads to the faces of all your clamps that don't have plastic shoes (and possibly also to those, in place of the plastic shoes).
User-made clamp pads can be plastic (although glueing many plastics to metal can be difficult), wood, ply or hardboard. I've even seen leather used but IME there are some problems with this that make it less than ideal and I wouldn't recommend it for clamps that will be used in glue-ups.
Some woodworkers use a form of double-sided tape to adhere pads to their clamps but on many clamps I've tried this on I've gotten creep, with the pads shifting off to one side when tightened very hard or slowly over time when the clamp has been left under pressure. Some clamp types will be much less prone to this but I always glue mine on now.
Scrape, file or sand away any paint from the metal so that the glue isn't bonding wood to paint but to the metal itself.
Even though PVA glue doesn't bond particularly well to metal you can use your standard white or yellow glue for this and the pads will be quite durable. A sharp knock (e.g. when you drop a clamp on the floor) can pop a pad free but it only takes a moment to glue one back on.
You can use superglue (cyanoacrylate/CA) but most superglues are brittle and you can have the same problem if a clamp receives a blow. Quick and easy option though.
If you want to make your clamp pads much more permanent use 5-minute epoxy.
1 Tips on how to achieve this can be found in this previous Answer. Be sure to click on the follow-on link.
2 Hot water makes a more saturated solution and like all chemical reactions it works more quickly at higher temperatures, so you get a two-fold benefit.
Notes on using oxalic acid
More than one application of oxalic may be needed for particularly dark stains. After three applications (letting the wood dry between each coat) if you're not getting any further improvement I think you can safely assume it has done all it can.
The rule of thumb is you treat the entire surface, not try to spot-treat the stain only (although you can concentrate effort directly on the stain) The original American Woodworker article the above before-and-after pic was taken from (issue #144, link here on the Pop Woodworking site) covers this, and it's also mentioned in this brief page on the General Finishes site.
Use appropriate caution Oxalic acid is a little toxic so should be used with care, especially when it's in the form of a dry powder where it can become airborne and can get in your eyes or be inhaled through the nose.
Cleanup After bleaching the surface must be left to dry thoroughly, and some oxalic acid will remain on the surface of the wood as a dry powdery haze. Don't sand this off, it should be rinsed away. Some old guides (and a few modern ones!) talk about neutralising the residue with vinegar but that's nonsense since vinegar is also an acid. You just need to use water, and plenty of it. If the item can't be held under a running tap rinse it at least three times, with fresh water each time. Then dry off the surface with clean cloth or paper towels and leave the piece to dry at least overnight.