Well, after some more many hours toiling and with the help of
\enabletrackers[lxml.setups] (Hans Hagen got to be thanked for that)
finally discovered how to do this. I report it here in case other
people might benefit. Maybe it should be put into the wiki, but I am
not very apt with that.
I cannot get multiple separate \xmlsetsetup's working. Obviously I am
doing something wrong. My idea was:
file included contains:
\startxmlsetups xml:commoncontext:commonsetups
\xmlsetsetup{}{error|tex|html|nbsp|...|br}{xml:commoncontext:*}
\stopxmlsetups
