Its a shame that the laws have you contemplating such a complicated arrangement, but it can be done.
250W would be fine on the front wheel. If you're going to burst @1000W then rear might be better. If you're sticking to 250W you could even think about powering your trailer wheel. 250W geared correctly would probably match normal pedaling acceleration and actually be more than normal pedal power at speed. It should be enough to match the acceleration of a slow rider, anyway.
The trailer is a good idea. I've often thought a bob wheel would be a good place for batteries and a motor. If its a bob trailer, make sure the top of the pivot axis leans toward the bike so that it will lean with the bike in corners. O-O\-O
With all the room on the trailer it should be possible to make your ICE motor whisper quiet. A fan cooled moped motor would be an option. SLA would work fine as the weight is not an awkward load on your regular bike and you would be using them as a electrical buffer allowing them to be kept mostly at a happy fully charged voltage. I'd think about utilizing a brake on your trailer wheel that would be slightly stronger than your rear brake, but actuated in parallel with it (trailer should skid first).
You have interesting options to have a motor installed on your bike, or not, if the trailer wheel is powered, or have the motor on your bike and the option to grab the battery off the trailer if you want to detach it.