Do you know how an older house incandescent dimmer works ?
Why are you saying that PWM control of an SSR controlled incandescent lamp would not work ? Have you tried it ? My recommendation is use pin 9 or 10 because those are 980 hz whereas all the other PWM pins are half that frequncy. Why don't you test it by wiring the SSR in series with the hot lead on a socket that has a power cord wired to it to plug in the wall so you can plug any ac powered device in it and dim it with the PWM. If it doesn't work, fine , at least you tried but don't jump to conclusions unless you can support your conclusion.