A couple of things:

Martin explains his string choices and gauges in the British Fingerstyle Guitar video/DVD from 1992. I don't have access to this at present but his normal tuning is CGCDGA and from memory I believe he says he uses standard D'Addario heavy-gauge strings with a .018 on the top string because it's tuned so low.

This would make the gauges as follows:
.018
.018
.027
.039
.047
.059

Because he tunes so low I don't think the heavy strings would require any special reinforcements or affect the guitar's warranty and a couple of things back this up.

Firstly, since 2002 he has almost exclusively played his own Martin Guitars 000-18MC signature model. The spec for this guitar doesn't mention any adjustments to specifically cope with the heavy strings and given that the specific model that Martin plays was custom built for him by Martin Guitars it's inconceivable that they would create something that would invalidate their own warranty.

Secondly, since 2013 Martin has occasionally played a PRS custom PRS Martin Simpson signature model that was bought for him as a gift by Simpson himself. A luthier friend of mine did some setup work on this guitar before it was given to him and at this stage it was already strung and tuned to Carthy's preference. When I asked what work he did on the guitar he just said a 'general tweak' with a re-cut nut being the only adjustment to accommodate the heavier strings.

Incidentally, on a (slightly) related note, for decades Martin exclusively used metal zither thumb picks but a couple of years ago he suddenly started using plastic picks which he continues to use now.