OK, final report (for the time being, at least): the "Japanese Timidity" doesn't work in Vista x64/Win 7 x64, no matter what i try. The solution suggested by robertmo does work in Win 7 x64. I still have to use "Putzlowischts Vista MIDI-Mapper" to select the default MIDI device, the other tool just displays one "unkown device" entry.
It may be interesting to note that it isn't necessary to change the default MIDI playback device in Windows. One can just create a special MIDI channel which is used by Timidity (if it's running), and point DOSBox to that "MIDI device". With this you can have the MS Synth as a default (which is convenient for a quick playback of MIDI files) and start the Timidity app only when needed, saving system resources.
Oh, and i didn't have to do any DOSBox config in order to get Timidity MIDI playback after setting it as the default MIDI device. That's how it's supposed to work, after all: "default device" should be the one receiving all MIDI data, and DOSBox is using that device by default.
All in all, i have to agree with Dominus. Having a "DOSBox integrated solution" (either Timidity or Fluidsynth - from what i can see both are similar) for MIDI playback would be very, very nice - especially because a) MIDI is so important for DOS games and b) correct playback is such a bitch to setup. Both Tim/FS are open source and multi-platform. Would be interesting to read a comment from qbix/wd about that subject.
To conclude: patotheduck, follow robertmo's instructions and you will get MIDI playback in DOSBox through Timidity under Windows 7 (should work in both x32 and x64 versions). Remember to actually start the Timidity application (the thing that creates the tray icon) and select "Start Synthesizer", otherwise all will be silent.