How would be the best way to temporarily lower the output of a 445nm portable? I'm thinking that the easiest way would be some sort of slip-on cap with an ND filter, but I'm concerned with heat generation (those extra mW have to go somewhere).
Maybe turn the pot down temporaily? If you use ND filters make sure it's glass, because as you said, these mW do have to go somewhere and plastic would get damaged pretty fast. You can always fit the piece of glass in some sort of drilled out heatsink.