It may sound a bit like an over simplification but I'd probably start with a DOS search for *.BAK and *.TRN files (*.DIF if you have those, as well) and just grab the name of the latest of each group for each database.

I've not personally tried it (finally upgraded over the weekend) but registering all of the servers\instances in SSMS and then using CMS to interrogate MSDB might also do the trick.