As suggested by Jens Petersen the package review tools such as rpm should be enhanced to check fontconfig files provided by packagers are well-formed. This check could take two forms: 1. check that all the files in %_fontconfig_templatedir are well-formed-XML 2. check that they conform to the current fontconfig DTD (this test probably requires for the fontconfig packager to register it in the system XML catalogs but Behdad is working on a new release right now so that's a good time to ask)
From rpmlint POV, I think a generic XML well-formedness/validation check would be a better approach than doing this just for fontconfig files. I've filed an upstream RFE about this: http://rpmlint.zarb.org/cgi-bin/trac.cgi/ticket/168 In the meantime (and even if/when implemented in rpmlint), I suggest font packages could add a %check section and validate their XML files in it - it should be easy for them to do it using xmllint even if the needed DTD is not catalogued (obviously as long as it is available somewhere locally).
Well, the problem with fontconfig is that its XML files do not use the xml extension, so you'll need to special-case their detection in any case.
Maybe ask fontconfig developers if they'd be willing to change the extension (if they're XML, why not *.xml)? Anyway, checking other stuff in rpmlint than just *.xml would certainly be doable. Added some related comments to upstream RFE, please add further comments there.