I think this is one of those things you should do some comparative tests with, see the differences with your own eyes on your wood, prepped as you've done it.
Without using dyes or other colourants to selectively enhance parts of the figure the general idea is to use a clear finish and generally one with some sort of yellow or amber colouration.
Just as you can use shellac for it you can use varnish applied directly to the wood to enhance chatoyance. Either one can give a very good effect. With standard commercial varnishes some believe particularly if the first coat or two are thinned (I'm not convinced this is necessary, I thin varnishes only to make them easier to apply).
But there are many who believe the effect is enhanced further if you oil the wood first and from what I've seen in my own experiments I would tend to agree. I do have to admit though that often the difference is slight, sometimes very slight. However, as oiling is only one extra step and doesn't take long I don't think there's any harm in doing it as part of your standard finishing regimen for this purpose*.
You can probably use any drying or semi-drying oil for this. BLO was the standard choice for many years among Western woodworkers. You can use tung oil instead if that's what you have. The two oils work very similarly in this regard, much more similarly than fanboys of either would have one believe.
“Popping” the Grain by Stephen Hatcher (PDF).
maximum chatoyance on Fine Woodworking Knots.
*As long as you're not trying to keep the wood as pale or whitish as possible in which case definitely don't oil.