I have a stupid idea.
Assuming Saturnine Centurions can take double twin heavy disintegrators - three in a unit of 10 tactical marines shoved in a Spartan. 12 S7 AP2 D2 shots and let the tacticals take the overheats.
If I've fucked up and they can't join tacticals, slap 'em in a unit of 5 barebones Cataphractii instead.