There's always the option of modifying the flash to take a lead to an external power pack. With some fabrication, the mod could be done so the connector fits in the flash's battery compartment and the lead exits through a slot cut in the door.
As always though, be very careful when fiddling inside flashguns - they can hold nasty big charges in their capacitors and can deliver a nasty belt! The concept I've described doesn't need the flash to be opened and the only permanent mod to the unit is the slot in the door. Even that might not be necessary if some other method can be found to keep the adaptor in place such as a Velcro strap or tape.