Hi David-
I tried unsuccessfully to get DV50 into PremierePro via firewire using that very same deck. Installing the Mainconcept DV50 codec gave me the option of exporting in that format but not capturing. I ended up doing the project on a Mac (FCP has native support). I was really surprised (and saddened) that PPro couldn’t do it. You wouldn’t have any problem getting the DV25 footage in, but the 50 is another story.
The only hardware solution I see out there that advertises DV50 support using PPro is the Matrox Axio, although it might be out of your price range if you were looking at Decklinks (I saw your post over there).
Sorry. I know it wasn’t what you wanted to hear. If you end up finding something that works, I’d be interested to hear about it…
Good luck. Sean