Does PowerGrep let you run commands/scripts against the output it generates? (Or could this be done via a OneNote macro? Presumably it has some kind of VBA or similar like other Office apps?)
If so, probably the best way to do this is to have the tool/script pass filename to an Opus command which adds that file to a collection. Instructions on how to do that are here:
--> Jon's post about user-generated collections
(FWIW, Opus 10 will make the command a bit more simple, but that isn't quite released yet. 
)
If there isn't any scripting involved so far, then you could make Opus run a fairly simple VBScript that looks at the clipboard and does the same thing. It would be very similar to my Paste File List button, except instead of creating each file it would run Jon's 'add to collection' command with the file path as an argument.
I don't know how much you are into scripting etc. so this may all be very easy or very hard for you to do yourself. Shout if you need a hand.