Should be pretty easy to do. As @peterboat says, it won't thermosyphon on its own without a pump as the hot bit is higher than the cool end (cauliflower), but it doesn't need a big pump. I've quite bit of experience with solar thermal on my boat. I would think that without a transparent cover over the photovoltaic panels you'll only get a significant output of warm water on the very hottest days. As the water in the cauliflower heats up, then the return coolant back to the panels will get warmer and eventually you'll run in to the reduced pv efficiency problem anyway. If the calorifier is already warmed up, then you might end up with reduced electrical output. Might be better to have the coolant going to a small skin tank to dump the heat in to the canal.
This may well be more expensive to set up than just adding an extra panel or two to compensate for the loss. If the panel array is sized to give you enough power during the summer, then on a hot day it will be over producing anyway. Twelve years ago I had a gadget that swung the two panels I had in an arc automatically to track the sun. Cost £250 to buy, but was worth it as an extra panel was £400 at the time. I eventually replaced it with another panel for £70 and mounted all of them flat to the roof and still got more output. Since then, panel costs have continued to drop.
Jen