I don't know how to create/use a patch, so I have edited my files manually...
for whoever may be interested, in order to check whether the url is internal or not (and return an error if the url is external) I have appended Ola's addition to /packages/acs-tcl/tcl/text-html-procs.tcl with this:
else {
if {![string match /* $url]} {
return "The attribute '$attr_name' is only allowed if
the source is internal. You have a '$attr_name' attribute
in there with the source '$url'"
}
}
