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Do any of then SNES emulators have a telnet/some other form of external connection mapped into SNES address space. Say for example to the EXT Port, so you can write bytes to it in SNES code and then have it be captured in another program? Maybe it could also be faked with one that has the fancy LUA script system?

Technically, Mesen-S has Lua scripting and has the LuaSocket package built-in, so it should be possible to do this? It's not exactly a full-fledged SNES emulator yet, though, which may be a problem depending on what you're trying to do.
I also haven't had the time to actually document the Lua API for it, but it's very similar to Mesen's.
Edit: To clarify, Lua scripting is available in the appveyor dev builds, but not in 0.1.0 (which is pretty out of date by now and really shouldn't be used)

I hear the nocash emulators have such a "debug register" that outputs to a window. It's rather simple to hook any emu's write function for a specific address.