We have done this as recently as last November, on Indiana Jones at DL. The whole party got FP, then when the time came up, we went to the FP return entrance, and traded my FP for a swap ticket that allowed me to enter the ride through the exit. I waited for DH just inside the ride exit, and as he came out, he took over kid duty and I went down the ramp in reverse. They put me right into a car.
Depending on how the ride's traffic flow is set up, the procedure may vary, but you can still do FP if you need to baby swap; you don't have to wait out the standby line while trying to ride herd on a smallfry.
We will occasionally attempt to cajole DS onto a ride that he doesn't feel brave about, but if he really doesn't want to ride (though he's tall enough), we swap. Lately he likes to forestall us on this. The minute we bring up one of those rides, he tends to sigh and say, "can't you just swap me?"
BTW, have they changed the swap procedure again on Space since Fastpass went in? Last time we used it (post-FP) the swapping parent had to wait in the arcade at the ride exit. There is no way in h*** I am ever doing that again! By the time it was my turn, I had a splitting headache from the noise, and from having to peel my toddler off of what seemed like hundreds of shiny flashing machines. I ended up not riding. I much preferred the old way, where they let you go through the ride line and wait by the chicken-exit door near the loading area.