Sooooo.... I am not getting anywhere with this SOYO board. It looks like either the Via MVP3 (586A/B) southbridge doesn't support D-DMA, the BIOS is bugged or the ETEQ copy of the VIA chipset isn't a 1:1 copy.
So I guess DSDMA is probably the only way to get the Yamaha YMF 7xx cards working in DOS on these SOYO boards and maybe all boards with the 586A/B southbridge.
I will try to test my EPOX board soon. It has the 596B southbridge so maybe it will behave differently.
If anybody else has a real VIA MVP3 based board with the 586A/B southbridge and a Yamaha YMF 7x4 based card that could test out the modified setupds that I posted in this thread that would be great.