Thanks for the reply guys!
We don't have the funds for a more permanent fix (digging up & replacing the sewer line) right now. I would've preferred to do that, if we'd been able to.
One thing I read or heard is that, even if you do that, eventually the roots will find their way in, even with pvc. (We've got 2 thirsty maples too close to the line). I'd be interested to know if that's true or not.
What my plumber wound up doing was installing a vertical insertion point a few feet up the stack, it worked well for the application. I took a few photos of the tree roots that day, and we're going to check it in November (6 month period when they should be gone, or at least less). Hopefully, if the stuff works, we can apply it yearly & not have to auger the line (and one day replace the line ;-)
As it turned out, the roots weren't totally blocking the line, but they were certainly going to keep growing through the summer.