After poking about a bit, I still haven't found what I'm looking for (with apologies to U2).
It seems that all the TImageList gyrations could be avoided if source data for images were in SVG format. Getting a particular resolution would be a matter of rendering the SVG data with the correct ViewBox dimensions.
I looked at BGRA, but did not see an ImageList equivalent.
Is anyone aware of such a beast, or have I found my next personal project that I really don't have time for?