Not yet. I'm writing it because of a new 7800 cartridge type somebody is working on and that work needs an emulator. I'm waiting for him to complete his half of the project before releasing it. If I release it now, it'll let the cat-out-of-the-bag too soon :-)
I have a fully working 2600 emulator that's publicly available. That'll give you an idea of what I'm aiming for. https://github.com/JetSetIlly/Gopher2600 Because the 2600 and 7800 are so similar, I plan to merge the two projects together in the future.