If I backup/wipe my hard drive, re-install pro tools, then replace the empty PT *edit*PLUGINS folder with the old one - does that work? Or will I have to install all my plugins again?
Simpler plugins that are self-contained and and dont use many external files (e.g., the digi plugins) can probably just be dragged into the plugins window. More complex plugins (e.g., convolution reverbs) and packages which rely a lot on external files, will likely need to be reinstalled. It varies based on manufacturer, as well. I would definitely recommend that you keep a backup of your current PT system and all of the plugins just in case you lose comparability with a newer version (which is not uncommon at all, unfortunately).
I have done this pretty successfully but remember that the folder to keep is not the plug-ins folder but the Digidesign folder that includes, not only the plug-ins folder, but also the plug-in settings folder etc. Otherwise you will loose all your presets that you have saved to the root directory, which tends to be the default setting.
This will save a lot of time re-installing most plug-ins. Remember that when you re-install Pro Tools it will put clean new Digidesign plug-ins so you might want to keep the new clean versions and only copy across the other plug-ins. But you will need to completely replace the plu-in Settings folder. You might then find that odd plug-in doesn't work properly, because files are elsewhere but again check your system before you start. I tend to go through a system and bundle all these kinds of files together into folders which I label so I know where to put them back. Some plug-ins put folders in the Applications folder, others will have files in the Documents folder. If you have samples, loops or impulse responses in a special place then make sure you make of copy of these too if they are on your boot up drive.
But overall you idea is sound and can help save a lot of re-install time. One final caveat though. Remember that if the reason for doing this whole clean and re-install is that you have a buggy system. The problem maybe due to a out of date plug-in so be sure that all your plug-ins are up to date before you start.