I like the idea of tweaking around the good ol' classical guitar... Nonetheless, never tried your "contrabass" tuning so I can't give a clue about possible tension issue. But I did try a less radical — althought very pleasant — low tuning, that I knicknamed "the poor man's baritone" : it simply consists in tuning the guitar from B to B, with a regular string set (omit the 1st E string) and a low B (Savarez and Hannabach both have these single strings, I found Hannabach to be more reliable and sounding less boomy) for the sixth string : this is a very lush tuning, full of beautiful overtones, that fits perfectly the music of Bach and Weiss for instance.
As for your idea, be aware that you might need to file the slots at the neck to accomodate the thicker strings. This must be done very, very, carefully in order to avoid the strings from buzzing, so if you don't feel like doing it yourself leave the job to your favorite luthier — and in any case, it's wiser to experiment these kinds of tweaks on a spare guitar or a beater.
Hope this helps, keep us posted.