This is extremely effective with low use cars. Also there have been various cases of cars with a defect (the Merc A-Class being one; we used to have one) which causes a few hundred mA to be drawn constantly (related to central locking AFAIK).
A normal size lead acid battery self discharges at a rate of roughly 50mA and even a very small panel, say 30×30cm, would compensate for that, even in daylight.
There are flexible panels available. I used to travel with a roll-up one – link – and that would do vastly more; it would charge a flat battery in half a day, enough to start the engine. So with some self adhesive variant one could glue it on the outside.
With no aerodynamic impact I doubt there would be certification issues, so long as you had a properly done battery connection (a fuse in the +ve cable, close to the battery) and disconnected it during flight.
If the plan is to disconnect it in flight, you might as well get a ready-made solar battery minder from the eponymous company for a whopping 180$ (though they carry only 12V versions atm).
I have a simple system in a vehicle of mine with two electrical systems. Apart from relays and solar and controller etc. I have simple permanent connection of the two via a 5amp light bulb.
This allows low current to flow, at high current the bulb lights up and the resistance increases limiting the current. If the current flow is too much it burns out.
I wonder if this could be adapted so you don’t need a controller or two systems. Assuming you looking for smart idea to make something cheap but safe!
My proposal would be incapable of overcharging the battery, even in earth orbit. But it would take out the self discharge.
People have been using solar cells to charge aircraft batteries for years, notably on Vari-Ezes that are so well adapted to hand propping that people tended to leave off the starter, which then led to leaving off the generator. Typically the cells are mounted on the top of the plane, connected all the time, with the hope of adequately keeping up with radio etc demand over the course of a day. Whether that works in practice I don’t know.
Are they attached to the plane and removed for flight, or are they fixed permanently?
Normally there is a curved surface, so it would need to be a flexible panel.
Actually the top of a wing may work better…