Classic A-190 + Shufflebox DIY kit = Swing and beat-synced Start/Stop
Combination of the classic Doepfer MIDI interface A-190 (or A-190-1) which is deprecated, but might be available on ebay, and the DIY kit shufflebox from frequencyresponse:
http://frequencyresponse.ch/?p=shufflebox
The button allows to stop the clock for the A-100 system against a steady external MIDI master clock, then restart it – both actions delayed to the next beat (quarter note). The poti allows to shuffle the clock. All non-clock MIDI messages pass through to the A-190. It works very well.
Requires MIDI sockets in the case. MIDI In from case goes into MIDI In from shufflebox, MIDI Out from shufflebox goes to MIDI In of the A-190, A-190 MIDI Out goes to the cases MIDI out socket. You may want to use coupling jacks between the modified module and the case terminals so that moving the module is still possible – one DIN 5 pin 180° connection using all contacts (=non-standard) does the job.
The modifications require medium skills. Hints:
No info about availability.
$272