This took too much time, so I hope it helps someone. Aside from the obvious use as a pad input and controlling the sequencer from the DAW, the BeatStep Pro can be used as a transport control for your DAW, the encoder knobs can be used as general daw/vst knobs for parameter control, and the BSP can be used as a Mackie control device for adjusting levels on tracks.
Attached is a zip file which can be extracted into C:\Users\<your_username>\AppData\Roaming\PreSonus\Studio One 3\User Devices (Windows) or ~/Library/Application Support/PreSonus Software/StudioOne 3/User Devices (Mac)
To prepare, start Arturia's Midi Control Center software, goto Device Settings
And scroll down until you find Mackie Control / HUI – set this to MCU.
Scroll up to Global and find “Auto-sync”, and be sure this is set to Off. This was not well tolerated in the DAW.
Scroll to Transports, and for the Transports option, and make sure "Both" or at least "MIDI" is set.
Remember to hit “Store To” on the far left to push the settings to your device.
In Studio One, Options->External Devices:
Add...
Browse to the Arturia folder which should exist.
Add Beatstep Pro Sequencer
Leave Receive From: None
Choose Send To: <your BSP's MIDI interface>
Choose all channels (most likely), choose the three midi options and hit OK.
Hit Add... again, and browse to the Beatstep Pro in the same Arturia folder.
Choose Receive From: <your BSP's MIDI interface>
Leave Send To: None
optionally choose 'default instrument input' or 'Split Channels' and hit OK.
Next, In External Devices, go to the Mackie folder and add the Control device.
Choose the alternative midi interface of your BSP - on Windows, this is called MIDIIN2/MIDIOUT2. Receive from MIDIIN2, send to MIDIOUT2.
==Sync==
If connected by USB, set the Beatstep Pro to ‘USB’ sync mode with the top-left ‘Sync’ button. Note that the tempo from the DAW is not guaranteed to be displayed correctly, but it is at the right tempo. Once in USB sync mode, freely recording your own pattern on the sequencer itself is restricted, so I wound up periodically flipping between Int and USB sync.
==Inputs==
Remember that your input for any track in the DAW, if it’s not ‘All Inputs’ will be the Beatstep Pro Keyboard, midi channel 10 for drums, or 1 / 2 for sequencer 1 or 2.
==Control the DAW or software instruments==
To use the device as a control surface (e.g. for midi learn/mapping) in Studio One, hit the Control Mode button on the beatstep pro, and choose ‘CC’ with the ‘knobs’ button.
To use the device as a Mackie control surface (e.g. to control track faders, mute/unmute tracks) choose the MCU/MUI option with the ‘knobs’ button.
Note that the device definition has an MMC feature, the record button, deliberately commented out / disabled. If anyone can make this work with Studio One 3 in a saner way, please update the device XML. The number of times it needed to be pressed as a toggle did not match up with Studio One's record transport command, and was causing lots of frustration. See Beatstep Pro.surface.xml
Enjoy!