Oops, I spoke too soon. There are some circumstances that do permit co-terminals. (Of course on the web site you can only book RTs with co-terminals if you originate at one; you cannot use the website to book a flight to co-terminal A with a return from co-terminal B.) The key is to use the promo code for your originating airport when you want to return to a different airport that is a co-terminal.

(I need to dig through my PMs to see what SWAV told me last year about whether RSAs "should" be able to book "web only" specials like this while using the co-terminal rule. If anyone knows, feel free to chime in.)

Side note: I sure was disappointed upon seeing that the travel by deadline for these deals was before LGA and BOS service begin. It's that much longer I have to wonder whether or not LGA is a co-terminal for ISP, and whether BOS will simply be added to the PVD/MHT grouping or whether the changes there will be more involved than that.

[Yikes, just noticed this is still sitting here in "Preview Post" mode. I thought I had submitted it about an hour ago. Mea culpa!!]