You could get t-tops on any model...even coupes. A friend of mine used to own a 4-banger 87 LX coupe w/ t-tops. What a cool little car!
Hell, you use to be able to get a sunroof on a coupe. I think 83 or 84 was the last year for that.
Oh also i think the first year Ford started doing the convertibles was 1994 with the SN95. I remember seeing an article in some Mustang magazine showing how the cars were shipped out to "cars and concepts" to have the convertible roofs put on and then shipped back to Ford. The cars in the pictures were Aero-nosed Fox Mustangs...so they definitely went past 1983 in terms of shipping them out.
C&C also did the t-tops, but 1988 was the last year you could get them, and i beleive those were only ordered unfilled from the 1987 model year. Not many made.
In the years before, T-tops were pentiful. I saw figures that from 81-86, 50% of all mustangs came with t-tops, while only 20% got sunroofs. That number dropped for 87 and even more for 88.
There aren't any definite numbers on t-top production.
Is it rare? No, is it desireable??? Yes