Have other people used e-bikes for touring? Seems like there would be logistical issues of the type you're hoping to address.
The extra weight of the electric propulsion system (18 pounds in this case) usually means that it's only useful if you can use all of the time. Otherwise, you end up carrying around 18 pounds of dead weight that is noticeably slowing you down (relative to the bare bike). Adding a solar panel (a few pounds?) in order to put a bit of charge back into the battery is just going to slow you down further.
I think there is a need to run a few calculations. If the desire is to charge the battery fully, you'll need a panel that can produce roughly 24v and provide 10A hours during the day. Without the ability to aim the solar panel, it might be safe to assume that you'll get full power for only 4 hours a day (a rough guess). That means that the panel will have to provide about 2 amps at 24v for those 4 hours (okay.... let's say 5 hours, to keep the math simple). The panel will need to be at least a 48 watt panel (24v x 2 amps), assuming 100% efficiency. Can you get a 1' x 3' panel that can provide 48 watts? Offhand, I don't know.
As far as a dynamo (like a hub dynamo?).... if it is only producing 3 watts, then it's not going to make a significant difference when it comes to charging a 24v, 10A hr battery. It would take 80 hours to fully recharge the battery.
Overall, my guess is that an e-bike is only practical if you use the electric assist all of the time. Otherwise, you're dragging around an awful lot of extra hardware that will really slow you down. I'd recommend putting 18 pounds in your panniers/trailer and seeing what that extra weight feels like. Is it worth carrying that that extra weight in order to get an assist for one or two hours?
