1. You can try to bury the rootstock including the trunk so that the top graft union (where the rootstock meets the grafted variety) is buried. You would need a very deep hole. I don't think cutting the top off and trying to plant it would work because there would be no remaining root structure to support that top portion.
2. You can try to take cuttings of the grafted variety, but if that variety is under patent, that would be illegal.
3. You could buy the bush variety of the plant.

I had three tree roses once during my very beginning gardening but they were never happy. There was a weak structure at the top of the bush but nicely formed so where the branches met on the main structure I lopped it off at an angle and put the three in a bucket with a bit of liquid kelp which I love to soak my cuttings in and in a day used Rootone, just a bit because too much will ruin the cutting, found out ftom expereince, and I have three very nice big bushes now. It's really no different from regular cuttings but the bases never did much and I never purchased another tree rose.

I think I would take cuttings, and also try to bury the remaining whole trunk.
Taking and rooting cuttings from patented plants, is only illegal if you sell them for a profit. To do so for your own use, from any plant you bought, or to even give them away, does not violate any patent laws.

Good luck with whatever you try.

Taking and rooting cuttings from patented plants, is only illegal if you sell them for a profit. To do so for your own use, from any plant you bought, or to even give them away, does not violate any patent laws.

This is incorrect. Any and all asexual propagation of a patented rose is illegal. It does not matter if you are selling it, giving it away, trading it or keeping it for your own use.