Hi Justin,
The 1908 Brazilian is a fine action and I wouldn't hesitate to build a 7x57 Mauser on one. They are beautifully finished and as slick as duck sh=t on ice. I have seen many 7mm Rem mags and .30 cal magnums built on them without issue. If you are concerned see if you can find via "Lord Google" any incidents of a 1908 Brazilian failures. I suspect you will fail.
Many years ago I had a 1909 Argentine that is supposedly a "soft" action. The front receiver was turned down to small ring specs, the chambering 280 rem, and I made a rather significant error with a reload. A 140 grn Barnes went thru the screens at 3450fps from a 22" barrel. Yes the lugs were set back a bit and I couldnt open the bolt at all, the barrel had to be pulled, but the action was completely fine. I suspect that there are too many intellectualisations made re the hardness and softness of various actions without much in the way of hard and fast data.