In any case, non-compliant files can still be hosted at PG, they will simply be labeled as non-compliant, and bugs will be entered into the issue tracking system indicating what changes will be required to make a file compliant.
My one suggestion here would be that PG not try to generate the EPUB and MOBI from HTML which is deliberately being submitted as being "non-compliant." You can make epubmaker jump through whatever hoops you want, but if the submitter is deliberately trying to write code that knowingly doesn't work on EPUB and MOBI, then again, epubmaker is being sent on a fool's errand. You are basically trying to make better design decisions about how to bridge the gap between large machines and small machines than the pros who design these small machines are making -- and that is exactly the business that the EPUB and MOBI [cough cough] standards writing committees are trying to accomplish, which is not something that PG can profitably second-guess. Again, another thing to think seriously about: Should perhaps PG simply "write off" support for MOBI7 ? This would hose many PG customers, but since MOBI8 is much more compatible to EPUB, and since EPUB is more closely compatible with modern HTML, this would greatly reduce the support burden, and would position PG to make progress to move forward in the future. I personally would like to see PG continue to support MOBI7, at least for a couple more years. But I can see an argument for blowing it off and just supporting EPUB and MOBI8.