The UART on the XT-IDE was for remote booting, it didn't actually provide a serial port at all. It was minimal so you could just get a virtual hard disk image onto an old machine, you couldn't really do anything with it.
The problem with SB/GUS/PAS is that those all use hen's teeth for chips or are far too complex to implement on a board like this. An AdLib can fit in the same space as a Covox Speech Thing. There's no reason you couldn't have an AdLib built into a card like this, and there's actually no reason you couldn't support both CVT and AdLib on the same card with memory - All of those parts are cheap, readily available, and through hole. Not to mention their designs are simple and do not take up much space, meaning that you could easily just stick them in with whatever else and it would be hardly noticeable.
An SB is a much more complex device, and a GUS/PAS/AWE64 are all almost full length and would never fit properly and still work right in that kind of space.
Personally, I have to disagree. A multifunction board saves me money because instead of buying 3-4 boards, I only need the one. I saw a video card with an MFM controller and a serial / parallel port on it once. I must acquire such a device, I can't really see the problem in combining them into one board. It's not like it wouldn't be period correct - AST SixPackPlus, QuadRAM boards, AST Advantage, etc.
Yep, HanJammer is right. It wouldn't be hard to make an exact copy of this card as it is, and I personally would be interested in buying one with the daughtercard. I'd like more than 2MB XMS in Windows 3.1, please.