A quick Googling shows lots of Citabrias with skylights. I don't know if they were standard on some planes or options, but it's listed in the parts manual as "greenhouse," so maybe you can add it to your 7ECA without a lot of red tape. It will take some fabric work, and you'll have to change some stringers, retainers, seals, formers, etc.
I think your best bet is to call ACA and see if they have a package deal on the parts. 262-534-6315. Someone correct me if I'm wrong on this: Since this is a factory option, you don't need a STC to retrofit your 7ECA if you buy the parts from the factory.
I'd be interested in installing one of these, too, sometime after I get my wings finished... So let me know what you find out.
-Alex