Hi dasimer, glad you like the plugins, always love getting feedback.
That sounds an interesting idea. Right now I know absolutely zero about the internals of an ePUB in terms of detecting a Calibre jacket but I'm sure if I looked into it or asked in the dev forum someone could give me a pointer. I know how to get the EPUB content (having done that for the Extract ISBN plugin), what I don't know is how to use that set of internal EPUB files to identify a jacket being amongst them. Perhaps there is a simple regex I can apply or similar. If anyone throws the answer at me I will add this sooner, otherwise I will put it on the list for a future release.
I'm going to be releasing a new version of nearly every plugin this weekend anyway to support Kovid's plugin API changes coming in 0.7.53, so if anyone has any further details on this or other ideas they would like added to Quality Check this week would be as good a time as any to see them happen...