If anyone's interested, I hacked together support for BibDesk bibliography completion using tm_dialog, without using osascript. I've posted it at http://homepage.mac.com/amaxwell as Completion.zip.
We'll have support for this in the next nightly build of BibDesk (BibDesk-20061123.dmg). If anyone's interested in trying it before then, and possibly giving feedback, I posted a build of BibDesk from current sources at the same location. I don't know Ruby, and suggestions on improving the command are welcome.
Source for the program is available on request; I'll likely upload it to BibDesk's svn repository as an example at some point.
regards, Adam Maxwell