EDIT: Got this resolved. See the spoiler. 
OK, upon adding it myself (because I like the idea), it's only half-working for me. For some reason, this fails to prefix:
Code:
characterlist=>^(.*)$=>Mass Effect.\1&&category==(Mass Effect: Andromeda|Mass Effect Trilogy)
(I also tried
&&category==Mass Effect)
But this works:
Code:
characterlist=>^(.*)$=>Portal.\1&&category==Portal
Did I make a regex error?
EDIT: Looks like a regex error specific to my setup.
Spoiler:
Commenting this line out helped:
Code:
replace_metadata:
category=>(^Mass Effect$|Mass Effect - All Media Types|Mass Effect - Various Authors|Mass Effect: Paragon Lost)=>Mass Effect Trilogy
However, the output matches what I put in as a conditional so I'm not sure what's going on.
Here are some fics I tested with
https://archiveofourown.org/works/17347502
https://www.fanfiction.net/s/7443161/1/
And this fic won't prefix even when I do comment out the line - do I need to escape the colon?
https://archiveofourown.org/works/28758381
I'll try messing around with that replace_metadata until it works again.
ANOTHER EDIT: I removed
^Mass Effect$ and it unbroke the conditional for the first two fics. Unfortunately, that's one of my more heavily-used ones. Any ideas what to do?
ANOTHER EDIT: I got it working!
Code:
characterlist=>^(.*)$=>Mass Effect.\1&&category==Mass Effect Trilogy
characterlist=>^(.*)$=>Mass Effect.\1&&category==Mass Effect: Andromeda