This looks like a long shot - a really long shot. I seriously doubt this will work properly…BUT, you never know…
There is no OS9 "MIDI over USB" driver. The standard class compliant protocols that exist today and are incorporated into your modern Macbook, iPad etc. did not exist back then. You are entirely dependent therefore, on FreeMIDI (since you're using DP) to recognize and communicate with the Beatstep over USB. The Mac USB Device Driver will "see" the Beatstep. It won't know what it is but it will probably pass data to it from DP if FreeMIDI recognizes the Beatstep as a MIDI device.
Without class compliant stuff in place, you're biggest worries are:
1. Will FreeMIDI even "see" this thing to begin with? And
2. Timing. Playing a sequence out from DP through the Beatstep to use its CV gates will be a breath-holding, fingers-crossed procedure.
The Beatstep is a modern device that is reliant on modern protocols to work - as I said, a long shot.