Couldn't hurt to crosspost this to filemakerjobboard.com and elance.com, or other freelance sites to try and find some help.
Since the google play desktop utility automatically monitors/uploads from designated folders, couldn't you just dump a container field's music file to that specified directory when ready for upload? Seems like a far simpler way to handle one way uploads, and saves a local copy as well (just in case).
The only caveat I see with that is making sure your embedded file tags (IE ID3 tags) have all the info they need for google. But if it's a music production house, they should be tagging their files in production anyways.
I'm sorry I'm not familiar with the UI you're referring to to upload songs from a designated folder. Perhaps that refers to upload of songs for personal use?
Nonetheless, I am quite certain this wouldn't work for what needs to happen for a record label that needs to make songs available for sale. Google is very clear on a strict format of their XML, required tags (prices, currencies, licensing terms, etc) and required folder structure hierarchy for data and media (audio, album art, etc) for each song and/or collection/album.
I've already been in direct contact with a couple XML experts in the FileMaker development community, who should be able to accommodate the client's needs.
ah yes, I'm referring to the client-side software. I wasn't really thinking about the "need to sell music" side of things, as I only use play as an online mirror of my music that's accessible via my browser as a streaming player. I've never actually purchased anything from it.