Hi Josh, sorry for the delay in replying.
What I'm thinking is this....
Option 1 - use xmp
Open the backup catalog into LR and select the images of each collection in turn, assigning a keyword for each collection name.
Write to xmp
Open the new catalog, select all of the files that might be involved, and read from xmp.
That'll pull the new keywords in (but will overwrite anything you've done to any of the selected images since that backup, hence my question about changes to any of the files).
You can then use the keywords to select the files to rebuild the collections.
Option 2
LR/Transporter is a very neat little plugin which can be used for this.
http://www.timothyarmes.com/lrtransporter.php
Open the backup catalog, select the files for each collection in turn, and get LR/Transporter to write out a basic text file list (.txt extension) for each collection in turn.
Open the main catalog, select all, and use the Mark Images using LR/Transporter command to mark all of the images from that text file.
Filter those marked images and create collection.
Repeat for each text file, each holding a different collection.
Does that logic work for you? Shout if you need more instructions on either scenario.