Finally managed to enrol at Miami. Landed from an international flight with AA at Terminal (Concourse D) and went through immigration there. Asked the officer about enrolment and he said I had to go to Terminal J. I was skeptical as surely that meant going landside (which I didn't think you could do for enrolment on arrival). Went through Terminal D, collected bags and went out to the check-in desk area.

Walked round to Terminal J and asked at the CLEAR security line, they directed me to the TSA enrolment center which, sure enough, is landside at Terminal J (near the end of the Terminal at Door 32ish). Went in, they had a sign-in list where you had to indicate whether you had an appointment or not (we did not). They worked through everyone, I waited maybe 10 minutes before being called and sure enough enrolment was completed. Given the set-up, I would say that you could probably enrol at Miami without either having an appointment or from landing internationally as the center is completely publicly-accessible, being closer to the street than even the check-in desks. They didn't even ask if we had just landed internationally (though I imagine they can see that when they scan our passport). YMMV may vary of course but this worked well.

Did this on Friday evening, alas AA's IT did not seem to process our newly-added Known Traveller numbers to put PreCheck on our boarding passes for Monday so had to wait in the 45 minute queue for security on the Monday flight home.