(I'm aware that this topic is over 1 year old)
Simon Kelley, the maintainer of dnsmasq is currently extending the cache to arbitrary type caching. There are a few minor details we are discussing behind the scenes with him, but this will very likely become available for everyone in the next released FTL version (it is already in the bleeding-edge branch update/dnsmasq).
It'll be an opt-in feature in dnsmasq for backwards-compatibility reasons but I guess Pi-hole will opt-in for you (no promises, pending internal discussions).
I will reopen this topic in case any related comments arrive.