Couple of thoughts there...
1) Sony is the primary manufacturer of changers in my experience, and they are FIRMLY pro-Blu-Ray, and wouldn't product a dual format changer unless Blu-Ray was 100% beaten out.
2) HDM takes up about 1/2 the room on my shelf... I know, still NEED a shelf, but I have almost 300 titles on a very small bookshelf.
3) There isn't currently a good, solid, well-featured dual-format player at all, so changers will be on down the road a ways.
Your best bet at this juncture would be the disc-storage units that let you retrieve a disc by typing in a number or looking it up with the attached computer's database, I've seen several AVSers use them, and they'd work with HDM just as well as CD's/DVD's.