What you want is libxml2:
http://xmlsoft.org/There's also expat, but it's not validating (only confirms well-formedness, not compliance with a DTD or schema).
There have been pre-built versions of libxml2 on one or more VMS freeware CDs, and the authoritative sources have VMS build support, though it may not be up-to-date. I have run it successfully on OpenVMS Alpha v7.1 as well as other releases in the current century. I don't know of any reason it wouldn't work on Itanium.
Calling libxml2 from DIBOL might be more fun than you deserve. Does DIBOL support ASCIZ (zero-terminated) strings natively? If not, you either need to concoct a way to build them or write wrapper routines in C that accept string descriptors, build ASCIZ strings, and then pass them along to the C interfaces. I've seen linked lists written in COBOL, which just means anything is possible with the VMS calling standard, but it doesn't mean everything is readable, maintainable, or easy.
As far as name mangling, libxml2 will have this problem to some extent as well, IIRC, because there are a few symbols that are longer than 31 bytes. One thing you can do is root around in the compiler-generated name mangling database (after building with CC/NAMES=SHORTENED) to see what the symbol names are that you'll need to use. You can also search the c.o.v archives for details of the polynomial used to generate the shortened names if for some reason you want to cook up your own method to generate them.
