i don't know specifically about MTX rims, but there is more to having a braking surface on a rim than just a flat spot by the rim lip. it is very possible to wear through the side of a rim with rim brakes so if the rims are not designed for it you can destroy the strength of your rim. check with Sun on the MTX thing before you buy (perhaps go for a rhino lite which is designed for a rim brake for sure)
edit: YES, MTX rims are rim brake compatible (i looked it up on the site, they have a machined braking surface.)
i don't know specifically about MTX rims, but there is more to having a braking surface on a rim than just a flat spot by the rim lip. it is very possible to wear through the side of a rim with rim brakes so if the rims are not designed for it you can destroy the strength of your rim. check with Sun on the MTX thing before you buy (perhaps go for a rhino lite which is designed for a rim brake for sure)