Some progress over the last few days:

've added basic Relax NG support to lxml.

lxml.etree introduces a new class, lxml.etree.RelaxNG. The class can be given an ElementTree object to construct a Relax NG validator:

>>> f = StringIO('''
...   <element name="a"; xmlns="http://relaxng.org/ns/structure/1.0"
...     <zeroOrMore>
...       <element name="b">
...         <text />
...       <element>
...     <zeroOrMore>
...   <element>
... ''')

>>> relaxng_doc = lxml.etree.parse(f)
>>> relaxng = lxml.etree.RelaxNG(relaxng_doc)

You can then validate some ElementTree document with this. You'll get back true if the document is valid against the Relax NG schema, and false if not:

>>> valid = StringIO('<a><b></b></a>')
>>> doc = lxml.etree.parse(valid)
>>> relaxng.validate(doc)
1

>>> invalid = StringIO('<a><c></c></a>')
>>> doc2 = lxml.etree.parse(invalid)
>>> relaxng.validate(doc2)
0

And in addition, I've improved the c14n support so you can produce canonical XML output for any tree:

>>> f = StringIO('<a><b/></a>')
>>> tree = lxml.etree.parse(f)
>>> f2 = StringIO()
>>> tree.write_c14n(f2)
>>> f2.getvalue()
<a><b></b></a>

The most awesome development so far is that there's a contributor!

He wrote a patch to support XPath extension functions. I still have to review this, which I will try to do soon.