Its not unrealistic because the IBF already do it.
Weigh in day before then again day of fight and they give you a 10lb leeway from the weight limit......... and so far there's been no hitches apart from that guy who refused the same day weigh in against GGG and could of been disqualified, the only reason he wasn't is because GGG accepted to fight him regardless but the size difference was clear.
There's no reason that can't be dropped to 3lbs day of fight allowing for natural fluctuation, hell even 5lbs over would be better than nothing.