I agree that breakage is not a good thing.
It is reasonable to issue requirements for "new users", i.e., in this
case things flowing in through various new directions. For the xml
direction direct to LaTeX outside of xmltex, non-TeX pre-processing of
CDATA to prepare for whatever you end up offering IMHO would be fine
so long as it can be viewed as a one pass universal filter for
chunks of CDATA.
My guess is that you want to stick with the current internal character