Are there any focus issues with a G70/HD-18 on a torus screen?

Depends on your ultimate maximum depth of the torus...I believe a 'torus lite' is the general consensus. A G70VR has better electronic adjustments for this application (and different lenses) and may be an indication of where a 'standard' '70 is less suited.

I find HD-18s hard enough to focus on a flat screen...

Are there any focus issues with a G70/HD-18 on a torus screen?

Sure you won't get excellent focus on the corners with a torus (short of lens's that were made for curved applications) depending how much curve you build into the screen. You can get it close but not as good as center focus.
I'll tell you comething else about the G70, mount it about 6" lower than normal on a ceiling mount application so that you can use less bow to cet the horizontal center line straight with less bow control.
But that said, I have done about 4 G70's with Torus screen and they all looked great.

Terry