I sort-of think that you should be able to "clone" TABLE to MyTABLE, and apply styles to that, and refer to constructs as MyTABLE and have it DTRT.
But to the extent the belief is, all tables stem from the one true canonical table style, this isn't such a bad approach. Find the way you need a table to look, and name it "Table" and then within the rules pandoc applies, that will be your table L&F
I too would have dived into the thing and hacked.
Maybe a tabular (hah) list of all the subtype instances you can overload, as a documented set would help? "To replace the default format for any of the following [A|B|C|D...] simply create an instance of this template data in the file <furble> and it will be applied instead" type thing with a couple of worked examples?
