Three thoughts on this:
1) The roofs in most structures are made to support specific loads over a wide area with no more margin than is required for there construction and in some places snow load. If you add 600 pounds at one point you may well have to reinforce the roof support or find a way to distribute it over 2, 3 or more joists to prevent sudden failure or to reinforce the existing structure of joists, rafters and purlins.
2) This brings me to the next issue which is failure mode... fail-to-safe or fail-to-unsafe. If the overhead support should fail (don't ask how I know) will it do a free drop to the floor or will it be cushioned by its rider?
You may need to consider some sort of frame that you role in under the hoisted bike for fail-to-safe support, to prevent free rotation and to hold the bike while you wrench around... gee sounds as if you're back to square one with a platform under the bike.
I use a lift similar to yours (cheapest I could get at Canadian Tire) and I had to make a fairly complex plywood adapter, using trial-and-error, that fit between the exhaust pipes. Essentially, the weight resets on the under side of the engine casing and is only mildly stable in the face of strenuous torqueing. Some, I hear, have made brackets to attach to the engine mounting bolts... but they will have to come forward with their inventions.
3) Remember, at the end of the day the objective is to have live, healthy and happy KZRiders ready for the road.
Best of success!