Doug,
It's not a problem with focusser travel. I have used my 10mm plossl in the PST and it is parfocal with my ToUcam.
I have only tried the ToUcam once with the PST, and it really wasn't ideal conditions. I started off focussing with the 10mm plossl, put in the ToUcam and then thought I'd fine tune the focus with the focus aid in K3. Well, due to a lack of detail on the sun, partly due to no sunspots and probably poor adjustment of the Ha filter on my part, and also possibly due to seeing, I couldn't really find any detail on the screen. I got the limb as close to focus as I could but it left a lot to be desired.
Because I couldn't get consistent detail, I couldn't really stack it very well either. Neither registax nor K3 could reliably pick the same feature
.
Having the PST on an eq mount would help because you wouldn't need to chase the sun so much. I've just been using mine on a fixed tripod so far.
There are two versions of the PST FYI. The H alpha which gives a nice bright image suitable for observing, and the CaK which gives a darker image so is recommended for imaging.
Obviously there are guys out there who can focus these things well - just look at Guillaume's image today! I just need to learn the knack I think!
Al.