Thanks to all who have contributed to this thread--I've just started using Topaz and love the effects I can get from it, but the last update created the won't-open-on-more-than-one-file-without-restarting-Photoshop error for me. (I am using Topaz Studio, which shows up correctly as a Photoshop filter for me.) I was puzzled at first as to exactly how to implement the transparency lock workaround, but I think I've got it working now and thought I'd add here the exact steps I'm taking.
When I have a layer in a Photoshop file on which I want to apply a Topaz effect, I first lock transparency in THAT layer, then duplicate it (the duplicate will also have transparency locked automatically), open Topaz Studio as a Photoshop filter, and apply the effect as needed. I save the layer back to Photoshop after adding each effect, and create a duplicate layer for the next new effect I want to add (every duplicate layer will automatically have transparency locked, of course). When I follow these steps, I can then use Topaz again in a different file (or as many times as I want in the first file) without having to restart Photoshop.
I do get this message every time I open Topaz on a "transparency locked" layer...
...but it hasn't caused any problems so I elected not to see the message anymore.