there doesn't seem to be a safe way of leaning an engine for 75% power
That's correct, but that isn't what you do. You first adjust the power** to get say 65% power or less, and then you lean for peak EGT.
So the fact that one of the pots is 50F ROP and another is 25F ROP and another is at peak, etc, doesn't really matter. However, if three of them are as described, and another is at 50F LOP then the engine will shake itself to bits because while the first three will be developing similar power, the 4th one will be developing considerably less.
** with a fixed prop, with the power lever; with a CS prop, with the MP lever
There is the caveat with a climb to high levels e.g. 10k-15k feet when you have to lean during climb because the engine won't run full-rich at those levels; again this is not an issue because the power is so low due to the thin air.