It's been a long time since I've messed with Applescript.
I modified a script I found on the algoriddim forums. This script would run through all tracks that were analyzed in djay and try to write the bpm data to the mp3 tag in iTunes. If your library is large, this could take days.
My modified script will take the currently selected tracks in iTunes and look for bpm data in the djay metadata plist file. Potentially a much smaller and more focused task.
[UPDATED 29 Feb 2012]
I have included support for manually-set bpm values, which are stored by djay in a separate plist file.
[UPDATED 29 Mar 2012]
I have included support for copying key data to iTunes, with translation of djay's integer value to string representation
[UPDATED 23 Dec 2012]
Bugfix: reset bpm variables in script to prevent old values from being held in memory for the next track. Thanks, Kieron Thorpe!
[UPDATED 22 Jan 2013]
Bugfix: tracks with a key of 8B-C were being skipped. Thanks, recoltonRobert!
Enhancement: warn user if no tracks are selected in iTunes.