It's alive!
I received the boards a few days ago and am happy to report that (almost) everything is working like a champ! I ran into a few issues along the way, but nothing major. One is that the diode is the wrong way on the silkscreen although I caught that before I actually soldered it in place and have already correct the board design. Another problem is that I can't seem to get MIDI output when using LSDJ as a master. The LEDs light up to the BPM from LSDJ so I suspect it has to do with the MIDI side of things. Just not sure what it is. I also don't have +5V working from the GB so I have to power it with USB still (I'm actually using a MintyBoost to supply power through the USB port. Works good enough for now).
I have only tried LSDJ, but mGB is on my list to do probably today. I am going to put together a full article on my band's website and I'll post a link here for the curious. I wanted to add a few things to make it easier for folks building their own (using the Shield design or otherwise), such pinouts for the link cables (which took quite some time to figure out), among other things.
Huge thanks to Trash80 because, damnit, this thing owns face! Once I've figured out the things noted above, I'm going to start work on an all-in-one design, which should save quite a bit of space and let me put it in a nice case, etc. But for now this is rocking out with my GameBoy out!