So here's what I came up with:
A piece of thick steel plate with a couple of angle brackets welded to it that will bolt to the frame mounting holes. The bike is very stable on the lift in side-to-side direction, but flexes a bit too much to my liking front-to-back. I noticed that my cheap Big Red jack is actually causing a lot of the flexing, so I may need to find a better bike lift before the next tire change.
I've never had any concerns lifting my 620lb Harley with this jack, but the Rocket seems to be pretty finicky to balance well, and removing the rear wheel will throw off any balance you had to begin with.