I did a slight variation on that when I pulled my trailer out to paint it and to change the bunk carpet. I wasn't getting much movement on the stern end of the boat, so I jacked up the trailer using two hydraulic jacks on the rear of the trailer. I then put jack stands under the boat, with wood blocking to distribute the load, and lowered the stern with the boat resting on the stands. I then jacked up the bow using a hydraulic jack with a cradle I built to distribute the load (so the weight of the boat wasn't resting just on a small section of the keel).
After that, it was just a matter of moving the trailer forward until one of the trailer's crossmembers was close to a jack or jack stand. I'd then jack up the boat behind the crossmember, move the first jack and roll the trailer forward. It really didn't take that long to free the trailer, and I was surprised how stable the boat was sitting on three support points. Some pics are below...
Jack stands supporting the stern:
Jack supporting the keel up front (not shown is a cradle I built that supported the keel after the trailer was out of the way):
Trailer freed with some help from my son:
I found the bottle jacks a bit easier to work with than using the tongue jack on the trailer, but I am sure I could have eventually got the same result using the trailer's jack.
Jim