I never had the nerve to push film, but I should have tried it. You had to tell the film lab, right?
I used to carry Kodachrome 64 and 200 in my bag, using both as might be needed, carefully rewinding so as to leave the tab out (I'd put my ear to the back of the camera and listen for it to come off the spool) and writing down the number of frames that had been used. Not much fun and fraught with disaster if anything went wrong. K-200 was a great film in overcast, a lifesaver. I'd go to Ektachrome 400 only in the worst weather.
Remember when SLRs could finally auto recognize film ASA? I think there was some kind of universal barcode on the film canister and maybe the film; I can't remember anymore.