Well, what you describe can be done, sort of. The trouble (as you found out) is that LR insists on treating everything its own and keeping it in a library. There are ways around this though. I do use LR (though I mainly use ACR directly from PM) as editor, but all my meta data changes are done from within PM.
Basically I ignore the fact that LR has a catalogue, I still manage and move my files around outside LR. This only requires me to synchronise the folders from within LR every now and then (otherwise LR won't pick up any metadata changes or new/removed files). See screenshots for a sample dialogue.
About the only other thing you need to be sure to do is have LR write its metadata changes to the xmp files (or do so manually). Oh, and of course have PM's metadata settings set-up correctly for LR (click the lightning bolt button in the iptc/xmp preferences, PM provides several defaults). Again see screenshot (no: you probably don't want to set them like I have!).
If you do this, both PM and LR can work well together. The only problem you now still have is that of PM not being able to see your edits from LR (except the crops). There's a solution to this too, but that means having to switch to .dng. (more on this in my
blog).
Hope this was helpful, if not, be sure to ask more questions
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