I finally found another way to get Cubase to "rescan" an XTC plugin. If you have a plugin that causes a "serious problem" when you try to save it in Cubase XTC mode, this can help you.
Initial problem was that things weren't perfectly aligned in DevicesXTC and VSTPluginsXTC (scopevst.cfg file, etc...) and when opening Cubase in XTC mode, Cubase grabs what plugin info it can get from the VST wrapper .dll file - which of course is incorrect, because misaligned .dev file (or whatever the cause) hasn't provided necessary info to put into Cubase's Plugin Information "Database". Simply restoring the proper alignment of VST wrapper .dll and Scope .dev file, naming, etc. does NOT tip Cubase to "rescan" the plug for updated info. Such plugins may very well show a 'zero' in the 'Nb Params' in Cubase's Plugin Information display.
Fixing VST wrapper and Scope Device files/names/locations WILL allow you to begin using the plug in Cubase, but you will get a "Serious Problem" message from Cubase if you try to save a project using such a plugin. Might even lock your computer!
Previous fix was to rename the VST Wrapper and the Dev file (i.e. MasterVerb Pro.dll(dev) become MasterVerbPro.dll(dev). This isn't always good - now you're using non-standard names, etc. Reinstall headaches, etc.
Just found that Cubase <i>will</i> rescan the plug without renaming as above, but instead updating the modification date for the VST wrapper .dll file. I did this by doing a command-level "debug" on such a file and the writing it right back to disk from there... Other ways to accomplish this I'm sure, but just re-copying the same file doesn't change modification date.
I hope this helps someone, is definitely going to save me some time.