To be honest, I wouldn't try freezing 'em. Sure, they may be ok to freeze, but the thing is... If u'r snake's refused it for a reason (eg, it's sick, has parasites, coccidea, etc.) You don't want these being transferred to the mouse. Cuz then you get u'r snake fecal tested, give whatever meds and it's all cleared up. Then a week later you defrost the previously attempted mouse, down it goes, and you've got a reinfected snake.
If you're just dangling the mouse in front of the snake, it has a sniff and ignores it, sure I'd think it may be safe, but if you leave the snake and mouse together in a tub for 10 minutes, that snake's gonna be crawling all over the mouse even if it don't eat it, so in that case I wouldn't freeze.
I know this can be expensive if u'r buying from the local pet store, and it happens long-term, but personally, I don't see it as being worth the risk.
And once you've defrosted a frozen mouse, and it's been refused, DON'T refreeze it, then you really must throw it out.