It would be cool to add custom arrows that can be fired by both regular bows and custom bows. Texturing such arrows while they fly is probably impossible, but we could texture them in the inventory. A long time ago, I did a small attempt to implement this, but there were quite some problems, and I was blocked by the problem that the minecraft client won't start pulling a bow unless the player has a regular arrow in this inventory.

Perhaps some kind of fake arrow system could solve this, but that would be nasty to implement because the inventory of the player would be full, and we would have to make sure that fake arrow will actually be removed. I think this attempt would be a dead end, but the custom texture systems introduced in later minecraft versions could be a solution: we can represent such arrows by regular arrow items.

This has been implemented in KCI 12