I took a look at doing precisely this a few months ago, but determined that I didn't have the time and expertise for it (I got XML::ValidWriter to install, but nothing much more). All I can say is, please, write it! It's too late for the job I needed it for, but future generations will still thank you profusely (assuming, of course, that future generations are allowed to see it--obviously not a given).

In my investigations (such as they were) I also came to the conclusion that SGML::DTD was the way to go, though you seem in any case to have done a more thorough job of researching the topic than I did. I do have some reservations about code that raises four different warnings at compile time, but it is somewhat aged (last change was in 1997), so perhaps that's to be expected.

Good luck!