Just an idea (again), if many people want a bit different things and it might be difficult to get the chips originally used in GUS (or other sound cards), why not make a FPGA board with enough sound card features like MIDI, joystick and audio connectors, onboard DRAM memory etc, and then design the sound card in the FPGA, so some might want it to be GUS, some might want it to be Adlib, SoundBlaster, CMS, Tandy, Disney Sound Source, Covox or whatever.
As long as the ISA card (or PCI card, or PCIE card) is designed and it has basic DAC, ADC, DRAM, etc chips, it could emulate any sound card.
Then, making a different sound card is just a matter of designing (describing the logic operations) in software and loading the resulting binary file to FPGA. Of course it could still have some sockets for actual chips if one prefers to use real chips in it instead of full 100% emulation (or for verifying the emulation is 100% accurate, or also have matching two decade old analog portion instead of high-end modern DAC and stuff.
Too bad I have not yet found a normal ISA FPGA board, many of them are ISA but in PC-104 form factor and have high prices as they are industrial IO cards.