Let me first go out of my way to stress that I don't mean this in any sort of aggressive or rude way, but I suggested letting FCPX put the media into the Library to make the archiving process simpler and more foolproof. From your OP it was sounding like everything was too hard. Now I can see you want to check your video project into a source code versioning repository, so simplicity is not the priority I thought it was.
It might work if you kept the media external and explained in a readme that whoever uses it will have to fix the symlinks to the media, then edit. But ... it's sure not what the product was designed to be used for and this is Apple - they will break any part of the Library structure at no notice if they feel it will improve the product and people who depend on reverse-engineered Library features will get no sympathy.
The XML is intended for interchange with other apps (colour grade and audio mix primarily) and I'm not up to speed on whether it actually does preserve the full, er, "goodness" (i.e. some fancy stuff may be lost) at the moment. I know in the *very* early days you would lose some stuff in the XML, but maybe by now it's a full representation. It is, at least, a documented feature so unlikely to get broken.
There are commerical MAMs that use XML and the media to manage project sharing/versioning/collaboration. Again, I'm not terribly current, but Cantemo certainly used to mention their strong FCPX integration.
Sure, it might be crufty, but the app really presumes a filesystem. If you want it all in S3 buckets, try DropBox*. Sharing with colleagues to date is more via SAN/NAS/external drive or some folks have been doing it with disk images. Xsan/external drives handle arbitrating who's able to write to the project, the others I'm not so sure.
--
*I haven't, as I said there's symlinks and voodoo so it may not survive. There's an old LACPUG movie online showing using DropBox and the (much less elegant) 10.x "event" structure to share a project. Was very much a hack, but one that they got to work