Mark,
Before I stumbled on the new Yanmar 1GM10 for a song via craigslist my plan was a parallel hybrid system using Thunderstruck stuff and a Chinese air cooled 10hp diesel. Did the measurements, found a spot for batteries, and determined I needed a small customization to one of their bracket/shiv assemblies to allow me to belt couple the diesel via a centrifugal clutch to the shaft along with the electric. Spoke to them on the phone and they confirmed they could make the adjustment, which really just amounted to adding a second belt shiv on the opposite side of the bracket from where the stock shiv was located.
When I was ready to pull the trigger and about to order their electrical stuff and a Chinese Diesel I sent them an email looking for a quote. No response. Several emails and phone messages brought me no quote, then I saw the Yanmar and went with that as a much cheaper solution.
I am keeping my eyes out for a 3-5KW motor for future use. I have worked out how to bracket it in and couple it to the shaft along with the Yanmar, and since I recently bought a welding rig I will just make my own brackets when the time comes. No rush, but it will be nice to be hybrid.
Much of the time for dealing with the in/out of a marina not much power or speed is needed, so even an underpowered electric drive is fine if it is not primary.