If weather is bad enough then yeah, they'll hold the PRT in the gym at Kay Hall. They do 20 laps around the gym for the mile and a half, which sucks for several reasons. First it's a LOT more cramped than doing it out on the track. Second, it's more boxy than rounded, so you have to turn four corners 20 times (my biggest pet peeve was people passing on the inside instead of the outside, so there'd be people cutting between me and the cone. And of course the DI told me to shut up when I tried to tell them to pass on the outside). Third, the surface of the court doesn't have as much give as the rubber stuff out on the track, so it can be tough on your shins (most people should be fine, since you're only running for 10-15 minutes). I was told to expect a shorter run time because of the shorter distance (supposedly 1.5 miles is closer to like 22 laps), but I actually added half a minute when we did ours inside.
And Jon, I don't think they'll make you run stairs. That could be pretty dangerous, especially as a group. This is an organization that makes you tuck in your shoelaces so you don't trip, they're pretty safety-minded.
When we had to do Reg PT indoors, they had us do it with our own respective companies, and since it was a strength and conditioning day, our PT body just led us in a bunch of exercises from the card. We just got a bunch of mats from the laundry and worked out in our own P-way. On a run day, they would just split it up between floors (or "decks," as they'll be called at OCS), and they'd do laps around the kill zone. I was UI on something else that day, so I don't know too much about how it went.