External cooling
If I were going to try this I think I would disconnect the seawater input line from the through hull and attach a garden hose directly to it. They should both be about 3/4". Clamp with a hose clamp, and turn on the fawcet. You will have to adjust it so you don't overfill the water muffler. I would be cautious about running the engine while on the hard with only a few jackstands holding the boat up. Hard to imagine what vibration forces you will have to deal with. Also why do you have to run the engine long enough to require coolant? Most of the cooling water is just bypassed until the thermostat opens. If you just need to run it for a short period, remove the impeller and start it up. Again not sure if I would start it on the hard to begin with.
