I haven’t yet had time to do more than a couple of hours testing, but it’s quite practical to use the gun in one hand and the roller in the other, following directly behind the gun. It also seems to work well, though it will need more practice, to wrap the seam tape over the roller so it trails behind, and use the roller in your palm and two fingers to guide the tape. This worked on one test on a flat surface, and with practice should work on a curved surface.
Also, you can tack-weld the tape in place at various points along its length, the same as you do with arc welding when you know things will otherwise buckle out of shape. Or start welding from the centre and move towards the ends, trimming to length when you get there.
Taping the two pieces together prior to welding is probably key, and finding that you can do that without melting the gaffer tape makes things much easier. All the heat goes into the tpu coating, and the other side of the nylon is not hot.