My OH is pregnant, and I've been looking on the highstreet without luck for a suspender belt that will fit her whilst she's preggers. Ordinarily she is a 12, and I've seen some here on LH like these
I cannot be of any help here but I thought I would respond to push it up to the top of the page and hopefully other members will be along soon to help.
I would imagine how much larger you would need to go would depend only on how much bigger she has gotten! Something like you posted would be able to sit underneath her bump anyway I think so perhaps not that much bigger?
I was in a high street store, (2 words beginning with "la s--z-" ) a couple of weeks ago and they had suspender belts at 50p - you could buy one of each size at that price!!
I have the iCollection one and it should fit her fine. The back is fully adjustable and the lace is wide enough that it would probably go over the bump as well as under it.