DIY midi daughterboards exist, however your idea of flashrom and large storage space is quite meaningless.

1. the boards are based on openly sold synthsizer chips(dream for example), which have their addressing limits, or even require to work only with certain specified sample sets.
2. midi port has a very slow transfer rate of <5kb/s, which makes loading customized sample set not really feasible.
3. even if it works, the performance is not expected to be up to the level of the old yamaha/roland ones.

So basically it's impossible to have flash programmable ROM (was thinking along the lines of USB to flash ROM samples and then installing afterwards)? Sounds like the synthesizer chip has to be specifically made for this purpose and none exist (correct me if I'm wrong).

Another question that I really want to know; can I use the gameport of a vortex2 (au8830) in real-mode DOS for MIDI? I've heard the only way for midi with this card is daughterboards in real-mode DOS. But daughterboards are extremely expensive for me and I'd rather use a genuine Roland-MT32 module (think SCB-55 is the only daughterboard for proper mt32 sound effects and it never shows up; Ultima Underworld uses water sounds for instance). Probably have to use analog Line-in audio in real-mode DOS as well (so midi module can be heard).

Not sure it'd work specifically for your purposes, but I vaguely remember some of the older E-MU synths having CF (or similar) expandability to allow you to load or modify libraries depending on the specific unit's capabilities.