Using a piece of 1 1/2 inch angle iron and 3 grade 8 bolts you can with a little time and a welder create an adapter allowing you to jack up a Harley Tri Glide with a motorcycle jack.
Photos attached.

 

What are the dimensions of your adapter?

 

It's only a thought but as you are only using one side of the jack do you know what its safe lifting load is?

Have you lifted the trike off the ground or just the back end? I realise if just the backend it won't be the full weight but they do weigh around 1200lbs.

 

The adapter is 6 inches wides and 6 1/2 inches tall plus 3/8 inches of rubber padding. Therefore the overall height is 6 7/8 inches tall.

 

Not true. The rear frame of the Harley Davidson Tri Glide is approxiamtely 7 inches recessed from the front The adapter is overall 6 7/8 inches high.
In the front section directly below the engine when the jack hits the frame. In the rear recessed section there is about 1/2 inch prior to the adapter hitting.
Therefore when lifting / jacking up the trike it is almost level with a very slight pitch to the rear.
I intentionally made as the slight angle to rear adds to stability.
The weight of the trike is disributed fairly evenly on the jack.
The jack that I use is rated for 1700 lbs and I personally never try to exceed more than 80% of a rating or in this case 1360 lbs.

 

You are 100% correct that lift the entire trike on one side of the jack would be extremely dangerous and un steady.