Dammit’s adventures in mountain biking

but if POC say "well, when we say waterproof what we mean, and what we think anyone else would take us to mean, is not really waterproof, but sort of waterproof", then I'm unsure what I can really do about it.

I suspect POC will rely on some given number for hydrostatic head (being able to withstand the pressure of a column of water X mm high without leaking). The MoD apparently accept 800mm, most outdoor clothing manufacturers will say at least 1000mm.
This is all well and good, until you increase the pressure on the fabric by e.g. walking on a groundsheet, or sitting on a saddle.

If you don't have any joy with POC, you could try adding extra waterproofing to the fabric in the arse area, maybe clear silicon thinned down with white spirit? This will compromise any breathability of the fabric, but I guess that's not the key function in this case.