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Yeah, mintty is too difficult (the user would need to use picocom or something to connect to the serial port). iTerm2 is MAC only...

RLogin also supports sixel & iTerm2 formats, and has built-in serial support. The default is in Japanese, but they have language packs.

Rlogin looks promising. One other requirement I forgot is that it needs to support x/y/z-modem, and Rlogin does.