Unless there is a space concern within the shop area, I'd keep the dust collection bin inside the shop, but run the PVC piping up through this crawl space. In other words, run the PVC directly up into the ceiling and then through the crawl space over to a central point where you'd have a down pipe going to your collection vacuum.
There are two potential issues with keeping the dust bin up in this space. One is space itself. A dust collection vacuum and bag are usually fairly large, larger than what a "crawlspace" would lend itself to. Secondly, if you can fit the bag up there, when it's full it's going to be big, bulky, and heavy. Getting the collection material out of there is going to be very tough. Even with a drop down stair, this is not going to be an easy or fun proposition. If you have a dust collection bag which is small enough to fit in the area you are talking about, you'll be in the position of having to clean it out on a very regular basis. You'll also have to ensure the roof structure is strong enough to support the weight of the collection vacuum as well as the load of dust when the bag is full.
On a separate note, if heat build-up is a factor in this space, I'd suggest you install an attic fan to draw out the heat. There are plenty of options out there for this and are fairly easy to install ... once you get past the idea of cutting a very large hole in a perfectly good roof.
