I looked into it a bit last night because I was bored. Plex does have their web API, but they discontinued their plugin architecture about a year ago.
It would definitely be possible to write a script or standalone application that syncs back and forth between MC and Plex, and if all you wanted was a one-time export, that should be possible to craft. There is even, apparently a python wrapper for the web API someone wrote (that was used to build
a system to sync between two separate Plex servers).
If you want "forever updating", ongoing sync though, that would be much more difficult. Without some way to get callbacks or push notifications from Plex when files get played, there'd be no good way to keep play stats synced between MC and Plex permanently. You could brute force it and just scan every file in the Library nightly, and if you use the old COM API you could get "instant" notifications from MC's end (so you could sync immediately when MC plays or stops playing a file), but you wouldn't be able to sync from the Plex-end in an efficient manner.
Best you could do would be to dump a recently played list out of Plex and MC, and then update those once per day or something at night (with a periodic full rescan every so often for stuff that got missed). That's what it looked like from what I saw in looking for an hour last night. Maybe I missed something though...?