After having no luck trying a different disk with the server OS, it looks like the Windows to Go install issue has been finally isolated to the USB stick itself (a 256gb USB 3.0 PNY Elite-x Fit microstick).
This was found by using AOMEI, loading w7 (with USB 3.0 drivers slipstreamed in) onto the microstick, then trying to use it to boot a laptop and a homebuilt PC, both with native USB 3.0. It wouldn't run on either. The w7 OS was verified as working by loading it onto a old USB 2.0 PNY stick and running it on the original USB 2.0 target computer (albeit painfully slowly as SIW2 said). But it would run.
It's still possible that part of the issues experienced is AOMEI standard doesn't play nice with integrating the Server OS onto the USB stick. Another potential issue is the potential for an unusable OS speed, even when saturating a USB 2.0 port with a USB 3.0 stick.
Some time back, USB stick distributor of what appears to be somewhat questionable quality sticks ran HD Tune on their 8gb USB 2.0 & 3.0 sticks in both 2.0 & 3.0 ports. Natch the 3.0 trounced the 2.0, but it does show a nice improvement using a 3.0 stick with a 2.0 port.
USB 2.0 vs USB 3.0 Speed Test Results - Premium USB
2.0 stick - 2.0 port /
HDTune Avg: 19.3 MB/sec
3.0 stick - 2.0 port /
HDTune Avg: 31.2 MB/sec (+161%)
3.0 stick - 3.0 port /
HDTune Avg: 54.9 MB/sec (+274%)
The only way to know for sure is buy a quality 3.0 stick with good NAND & controller that will run w7 on a USB 3.0 computer, then take it from there.
So the question is, should I be looking for a high end stick with a SSD controller on it (something like Corsair's GTX) or is that overkill?