Since no one answered this, I figured I'd report my experiences. Maybe it will help someone else.

The major issue for a non-Portuguese speaker is finding an agent at one of the mobile carriers' stores who speaks English. I focused on Claro and TIM because they have portions of their websites in English, which is helpful for things like dialing instructions. TIM ended up unable to sell me the "TIM Visitor" plan (no explanation given) and I also couldn't buy a regular plan because of some technical problem that was temporarily preventing pre-paid cards from being recharged with credit.

So, after asking at several Claro stores and being told to go to different ones (ultimately the one on Avenida Treze de Maio in Centro, near the Carioca Metro station, had someone who could help me), I ended up with a chip, R$30 in credit, a nice internet data plan sufficient for my stay here (including free Whatsapp data, which is fantastic because that's how I call home), and a barrage of Portuguese texts from Claro in which the only word I understand is "Disney." (huh?)

The Claro chip is also nice because you can recharge it right from your phone using a credit card. TIM doesn't allow international visitors without a Brazilian ID to recharge online. So far Claro has had excellent coverage in Rio, and even here on Ilha Grande!

Edited to add: As far as I can tell, there are no SIM card vendors at the airport (GIG).