As Bubbless already said, it is possible even on Jolla (even if it is not very comfortable in some situation, with a small chance to drop your phone because of the finger achrobatics required to do that; I blame the phone dimensions, it's that little too big). But even with the current dimensions, you would just need to have a slide input keyboard and voilà! Typing with one and only is not only possible, but really convenient. Oh, how I miss it from N9... when it was enabled in... what was it, PR1.2? It completely changed my experience with the device, so, so comfortable.

Regarding the rest, I strongly disagree with anybody saying Sailfish is not designed with one-handed operation in mind, imho it's totally the opposite. Event view and peek gestures, putting apps in background, accepting/refusing actions, accessing the pulley menu... aside from some small quirks while catching the input desired from the user (especially with the pulley menu) all of these can be easily performed with one hand. The only gesture difficult to perform in one handed operation (and it was so also on the N9) is the swipe-from-top to close an app / lock the device. But again, I blame the dimensions of the phone: on N9 it was a little bit more difficult to perform than the rest, but still doable; on Jolla there's the serious chance to drop the phone while stretching your hand to reach that top spot, and it is also not that comfortable. That's the only gesture I use my left hand for. That, and typing very long messages (a thing I hope can be solved soon with a slide input keyboard, but development on both open source implementations announced for Sailfish seems to be stopped.

EDIT: wait, the issue descibred from nodevel in the post just above mine is also true... and the proposed solution is brilliant! Please, do it!