Could you get around that block by finding the biggest directory?
MusicBee > Main Panel > Tracks > Right Click Column > Set Displayed Fields > Path
Then open that directory in Windows Explorer and collect the 87 bpm tracks
Windows Explorer > Right Click Column > More > Beats Per Minute
Drag those 87BPM's to a temp folder, then add that folder to Mixxx?
I know you said it's uninstalled and over with, and I'm kind of interested in attempting this with my laptop. But I'm worried about time. Maybe my laptop is slow, but when I used MixMeister to analyze 3000 bpm tags, it took a day and a half. About the same for Mixxx. How long did it take you to analyze your 40,000?