If I am not mistaken poco doesn't have xml scheme validation.
Would an implementation using MIT licensed libxml2 be possible?
In fact libxml support all and more than the expat library.
Wouldn't replacing expat by libxml2 be worthwhile?
libxml - http://xmlsoft.org/
latest release - ftp://xmlsoft.org/libxml2/
Example using libxml from a C/C++ Application - https://collada.org/mediawiki/index.php ... validation