First off, the solution that is coming is more in line, I believe, with how SOngs work. IF I open a service file on a computer using a different DB, any songs not in the DB are added on open (unless the feature has been turned off) and I can work with it as tho it was always there. That, as you noted, doesn't work the same for Custom content. And I've found that it doesn't work perfectly as you describe in most contexts..
I share the OpenLP data folders across computers using Cubby, better deal than Dropbox, since I have no *nix boxes at the moment. If the worship leader/pastor/someone adds a custom item on their computer, and puts it in the service, it LOOKS like everything happens right, but I cannot edit the custom item in the service still. Not sure WHY it doesn't work, but it doesn't. At least not as of one version back, I haven't had a chance due to time and illness to update to the latests version across the spectrum, because there's a lot of instances to update..