I would like to use this chance to ask what is MESS stance on non-commodore ROMS, i.e. IDEDOS, JiffyDOS, DolphinDOS, etc. I understand ATM would be futile to add them since bug reports might become unnecesarily complex, but I would like to know if they will be added in the future.
once everything else is working, I'll take a look to these
I personally think supporting third-party BIOSes is the worst idea since N64 HLE but many MESS drivers already support them.
unfortunately, with computers is not as easy as with arcades. e.g. ZX81 supports as BIOS variants two different implementations of Forth, both never produced by Sinclair. One was available as an add-on for schools which wanted to use a ZX81 to teach informatics, the other could have been ordered as a chip replacement to the producer. Therefore, for both we are quite close to the situation of some obscure bootleg included in MAME...
Moreover, to install new unofficial components on home computer is way more common than it is on arcade PCB: sometimes imho it's quite hard to decide the threshold
There are also a few BIOS which could be close to homemade, but in MESS we have a flag for modified computers, so in the worst case one can leave the parent with the 'official' BIOS only, and create a clone (with such a flag) with all the homemade BIOSes...