It's more complex than you seems to think. What about catalogs for example.
I'm sympathetic to any request toward adding security to libxml2 but
like any good security framework design this has to be done globally and
discussed with all parties involved. Joing the mailing and raise the
issue there with explanations of your rationale for this. Nothing would
be worse than an option giving a feeling of security when in fact the
framework doesn't provide it. As such the request is incomplete. And I ask
this to be discussed on the mailing-list, using bugzilla to request just
one bit of it without even an use case is not proper IMHO.

Daniel