Hi Vince,
I don't know of one being made yet. If you decide to make one, I like to suggest something about the uart for the serial port. You might want to make the serial port compatible for several different types. So that anyone with a S-100 system can use this and still be software compatible with their system, Dos, CP/M, etc.
There are about 3 to 4 types which comprised most of the software from back then. I've had to patch software to make the serial card compatible, but not everyone has this option. The original Solid State Music IO-4 was an attempt to do this, but shortly after more software was written for these other uart chips. It was compatible with the Altair, Imsai, NorthStar, and a couple others.
I've been collecting info about the S-100 cards on my webpage here at the below. I was always looking for data about this but it was always scattered about on different websites, on my harddrive, but not online, so I consolidated it all onto one webpage - it's still a work in progress -- which I have to get back to.
http://www.vintagecomputer.net/ragooman/comp_s-100_serial.html
I started this webpage first because the serial card is the most common starting point for upgrading a system beyond the front panel. The next page will be about the Floppy controllers. This is part of the CP/M workshop material from the workshop we had back in April. This will eventually be online too.
There could be different approaches to this. One is to wire each type directly on one board -- simple but space consuming. Maybe another approach is to write an emulator using one of the AVR micro's which you can select the type with a Dip switch.
=Dan