@inspexor Possibly I'm mixing topics here so assume this source is unreliable.
I've had good luck saving skirmish games after launching with init_coop.lua
So far I've not been able to save and load a regular campaign mission. I can't remember the exact behavior now - it has been ~6 weeks since I tried. I'll try to add links to a couple of threads that looked pertinent to me.
link load-save-broken
link load-save-under-repair
But my attempt was with the regular campaign. You mention a custom campaign so your experience may be different. @deletethis should have a better idea than me.