I am thrilled to report that a few weeks ago I was finally able to capture this odd relative construction that my grandmother occasionally perpetrates in Polish. The "standard" phrase would run something like this:
Tomek, którego poznałeś...
Tommy-NOM, which-ACC meet-PAST-2SG
"Tommy, whom you met..."
Really a very normal Indo-European relativization strategy. The somewhat weirder one that my grandmother has been observed to do is as follows:
Tomek, co go poznałeś...
Tommy-NOM, what him-ACC meet-PAST-2SG
"Tommy, that you met him...," i.e., "Tommy, whom you met..."
So instead of a declined relative pronoun, we have what may be an invariable marker something like the English "that," followed by the relative clause with a retained pronoun. I have never (consciously) observed pronoun retension in Polish relative constructions in any other context.
It would be interesting to find out how common this is, and whether that "co" is really indeclinable in this construction -- under ordinary circumstances, of course, it has the full array of forms: co, czego, czemu, czym, etc. I'll have to try to come up with a test that I can administer to my grandmother without making her self-conscious.
Interestingly, by the way, my mother's usual way of phrasing this kind of relative clause in English is a nearly perfect analog to this Polish clause: "Tommy, who you met him..."
Other comparative linguists out there: how often does this pronoun-retension strategy manifest itself in Indo-European? The only IE language I'm aware of in which this is the standard is Farsi, if I'm not mistaken.