Thanks for the response!
No I haven't. I actually thought of it, but it wasn't an option...
(it would have implied some complication on the build process, that
the coworkers wouldn't have agreed on - they preferred to just
ignore those red flags)
I really just came back to post that I did find a solution
myself. (well, it was behind one of those google-hits, that I
had previously discarded as irrelevant by the abstract :-} )
For the record: In the Window->Preferences dialog, I pick Validation,
and in the line "XML Validator" I click the triple-dot-button. In the
Window that pops up, I then click on "Exclude Group", then button "Add
Rule".
I then selected the folder that contained those purposefully bad
xml-files, but I could just as well have selected each file indi-
vidually, or (probably) specified a pattern like "testcase-42.*.xml"
for the files to ignore.
It didn't immediately show any effect, though (even after "OK"ing
all the dialogs up to and including "Preferences"). I then right-
clicked on the folder in the "Project Explorer" pane of eclipse,
and from the context menu I picked Validate. Only then, the red
flags disappeared.
Nice thing is that (unlike renaming the files) these preferences
only affect me, so the coworkers still see the red flags on those
xml-files in their eclipse...