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?