Solar boilers might actually be harder to automate then fuel based ones as there are no automatic valve releases the free steam becomes a liability as the sun would be constantly going perpetually building up pressure. With fuel based boilers you can calculate the right amount of fuel and place it in without fear of your rupture discs going off.
Except there's so many ways to create automated outlet valves already that it would be trivial, unfortunately.
I think the idea is actually interesting. The way I would tackle this one is by measurring pressure and making a horizontal piston door to block of the sunlight. If the solar boiler was made to be a 3x3 multiblock that would become more challanging.
I realy dont know why mod devs insist on generators needing fuel so much. Especially because there are so manny mods that trivialize getting that fuel... I'd much rather have a multiblock that requires some thinking than going: "yeah, this mod needs a tree farm, this mod needs a treefarm, for this you pump the nether and for that we build a MFR laser". Its boring, realy.
And here's the thing, such a multiblock does not need to be just power. Red Power had this generator called a thermopile. It would generate power based on how much heat and cooling was around that block. Lava would heat it, water/ice/snow would cool it. With snow/ice being better then water. The cool thing about this? If the thermopile was cooled more then heated it could turn lava in obsidian or flowing lava into basalt. Suddenly you dont just have a power generator, you have an interesting mechanic that, when enginered right, is also a cosmetic block generator. It was one of the most fun projects ive ever made in modded minecraft (even though the power output was only ~25mj/t
)
TL: DR Id rather have a generator with some unique mechanics then a generator requiring an easely automated fuel.