I just compiled WiiMC today following this tutorial as I also wanted to be able to use WiiMC with UStealth.
I modified, or more accurately, removed line 570 of fileop.cpp as suggested by Cyan and got rid of a couple of brackets as there was no longer an if statement. This gets rid of the MBR Signature Verification. Hopefully this is sufficient and I haven't caused a bigger unseen problem. I have tested it on my Wii U, however, and everything seems to be working so far.
Here are the compiled boot.dol and boot.elf. Once again, I'm not sure if I made some mistake making the necessary edit or in the compilation, so use these at your own risk. I haven't done anything with code for probably five years, so it's a distinct possibility. However, following the tutorial I linked to it should be possible to compile your own modified version, even for someone not familiar with development environments.
Good luck, and if anyone decides to use this (besides me), I'd be very interested to hear if it works.
Awesome, thanks a ton - I had hassles getting wiimc to compile. I'll add your compiled project to the OP.