Re:
Rob, I've not had that rim and tyre combo but don't think it's a problem with the rim as such. Tyres themselves are designed for certain limits of rim width, stretch a tyre over too wide a rim and it will lose all it's compliance, too narrow and it will be unstable. Check with the tyre manufacturers to find out the rim widths they are designed for, to see if your rim falls within that limit. Think the rim manufacturers are just suggesting tyre sizes to use on rims.
The only thing I can think of concerning the rim itself is that, being a rim-brake rim the leverage of a bigger tyre on rim wall might be a factor.
But I think that's a non-starter as bigger tyres tend to be inflated at lower pressures so less leverage on rim wall. MTB tyres aren't usually inflated to high pressures compared to road tyres but even then unless there is rim damage due to brake wear I don't think you'll have a problem.
I have seen 2 rims break, both on my wife's bike, due to rim wall brake wear not tyre size.
