It turns out that my wrapper class has got to work with Microsoft's SQL Server as well... and SQL Server's syntax for XML operations is different to Oracle's. Hauntingly similar, but not quite the same!
Since what I needed to do was quite straightforward, it was reasonably easy to work out what the equivalents were... well, after several hours of wading through Microsoft's help pages and endless experimentation with sqlcmd (SQL Server's equivalent of sqlplus).
Finally, I was almost done; the last operation I needed to do was giving me a somewhat unhelpful error message. I Google'd for it, and got exactly ONE hit... a guy asking why he was getting this particular error... in German! So, crossing my fingers, I read the response—also in German—and as I hoped the SQL snippet supplied was in XPath/XQuery-standardised English notation. That gave me the answer I wanted, and now (after some screwing around) it's all done. Firefox's translation extension did a reasonable job of interpreting the posts, but it wasn't actually necessary to work out what was going on (luckily).
Ah, what fun! Oh, for AI-assisted programming... or at least an AI-enhanced help system! Anything would be better than the usual keyword-based search that is pretty much all you get at the moment.