As you suspect, if it cant be turned manually, the starter shouldnt be used.
I have heard and read of accounts where guys did an initial upper cylinder lube, and then went out each day, manually trying to turn over the engine. One way, then the other, gaining a slight amount of movement each day, finally to break it free.
Personally, I think it is most of the time a waste of time. The times I have done it was with engines out of vehicle. The crank had to be turned to get the engine disassembled, and the cylinders were always a mess. Just take the intake and heads off and see what the problem is. Could very well be a broken head(s) which leaked coolant into cylinders, to sit and rust for years. Or if the engine sat exposed without hood or air cleaner, same deal. Or all could be ok. Then it is an opportunity to clean up rocker and chambers, install gaskets, and see what happens.
One home brew for a WD-40 type lube is 50% ATF and 50% Acetone. I've tried it a couple times on various things and still dont have an opinion on effectiveness. One thing for sure, it has to be shaken immediately before use because the concoction separates very quickly.