Emulation!? Heresy! 
I might be mistaken, but I’m not aware of any emulators which have yet been adapted for MIDI (or some hardware or virtual linkport alternative which might be adapted in this way) outside of dedicated chipmusic software and plugins (which are not designed to run ROMs), and I would have to imagine this to be the case with the GBZ, based upon a cursory search. It might not matter for your intended use, but it’s something to consider if you’re hoping to make a more robust instrument out of the project.
Well, the Pocket wouldn’t be at all worth the price but for the FPGA, of which it has one dedicated to the Gameboy and other handheld systems but also a more powerful FPGA for developers to tinker around with (this is perhaps the biggest draw for me).
For my part, I get a bit precious about the original hardware, apart from their FPGA alternatives which make me salivate profusely. If you get the itch, there’s plenty of DIY enthusiasm in the realm of FPGA-based classic hardware reproduction (though unlikely to be made to fit in a DMG shell or comparable device, I think, without some commercial-level electronic/industrial design chops):
Edit:
The GBZ’s prospective uses remind me of the way I’ve occasioned to use my PSP with custom firmware for LSDJ sketches, in that I would either just use it on the PSP for more off-the-cuff music-making or else for starting work on a song that I would finish on the DMG. I also used to use that for LGPT and believe that something like the GBZ would actually work way better for LGPT, since you could make use of MIDI with that software/hardware combo.
