Say I have a lot of files that I want to add music or room tone or whatever to.
Target files are different lengths. Would it be possible through watch folders / batch processors to have WL mix a given file into every file?
Long shot - I know. A not easy because they have different lengths.
Hmmmmm
Never mind different lengths - my understanding is that batch processors can only act upon “finished” files. In other words - you do your mix in whatever tool fits the job and then feed that “already mixed” file to a Watch Folder/Batch Processor.
WL’s Batch Processor has no ability (that I know of) to combine sources and mix them on the fly…
VP
I also don’t think WL Batch Processor/watch folders can do this but I can’t be sure as I just don’t have the need for it.
There is a new batch processor app called Myriad (formerly Sample Manager) by Audiofile Engineering. It’s still in public beta (January release they say) but I believe it has a function that might do what you’re looking for.
http://www.audiofile-engineering.com/myriad/
There is an action called “mix” that could do what you’re looking for. I’d sign up for the beta and see if that’s the case.
Maybe PG could consider your request for the future if it’s not already possible. I’m not sure how widespread the use of this would be.
I know - theres a batch function called audio injector that puts in audio on the beginning or end of the file and that’s probably the closest I get.
I was kinda hoping I had missed something obvious in the batch processor or montage so somebody would go “that’s easy just…” 
Not in WaveLab, but if you’re not averse to using a command-line tool you could look at FFMpeg, specifically the amix filter. FFMpeg is free and open source and is basically the Swiss Army knife for all things audio-visual, however it does have a steep learning curve if you’ve never used anything like that before.
This will do everything you need and will also account for the varying file lengths.