I would put a bit of the shiitake you use for the dashi...the bamboo sounds good...you can live without the ginko (though that is my favorite part) but you need something with a similar texture...the fava is OK but a couple edamame may be better. Something sounds odd about putting tofu in there. Perhaps the texture is too similar to the chawanmushi itself? I have had some with a tiny piece of some roasted chestnut. But whatever you do, remember that the custard itself is the main attraction...don't load it up too much with other stuff.
Hmm.. after reading this, I realize my plan made my inherent gaijin-ness obvious.
I will do some edamame, the shiitake from the dashi, bamboo, and maybe something else. Small amounts. I've heard that the awesome part of chawanmushi is that the custard is so great and a great platform for all the little treats inside.