Do you know what actually is happening with the affected horn growth you refer to? It seems to me the dryness may be causing the keratin to curl down at the edge as it grows causing each successive growth ring to be lower. Doesn't seem like pyramiding is actually upward growth of a pyramid but downward growth in between - based on measurements of overall shell height vs length.i think that is the best conclusion i've seen , it would satisfy any argument i've seen ......... it's similar to what this Weisener and Iben guy were looking for , except when they start talking about dietary calcium , phosphorus and ossification , i personally don't see a connection ..... http://africantortoise.com/_sulcatadiet2.pdf
don't know about the type of keratin a tortoise shell is made of , but the composition of keratin in horns is known to be affected by humidity , i would assume possibly the keratin in a tortoise shell is also , and that very well may be related to pyramiding ..... your conclusion would also answer why the introduction of an artificial food source in madagascar would lead to seeing pyramided wild radiated tortoises ......