That would also be my suggestion: select the "XML" syntax definition in the File Properties popover. Textastic will use this syntax definition for all files with the same extension after you do that.
Textastic highlights with different colors the various parts of the html or xml code (tags vs. data). It doesn't do that for xsd files; they're all the same color. Even in html or xml files a mismatched tag isn't flagged (<head> vs. </head>). If I remove the / from a tag it isn't flagged as an error by changing color.
What I was envisioning is a way not only to help code and verify the tags in the xml data and xml schema (xsd), but to also validate the xml against the xsd like the web site below does. It validates that the xml conforms to the xsd. You can upload and validate XSD and XML files from a PC on the web site, but you can't upoad files from an ipad. http://xmltools.corefiling.com/schemaValidate
You can also add your vote for w3c validator support at http://feedback.textasticapp.com/topic/57678-w3c-validator-support/