If I find a mod that does that depending on how it's implemented then it would possibly be added. Meaning finding one that works for the version of vB we have and it can be enabled on a per usergroup basis so only Patrons could use it.

Until then I'd suggest using the folder feature and moving PMs to specific folders which makes finding stuff later easier. If that won't work you can download your PMs (links at the bottom of the PM page) and search them on your computer.