Though i believe your 3pt lift would lift the weight 9 or relief.. either way.. not hurting anything), and if the trailer does have a tounge weight of 500#+ sounds like a big trailer.. like a 16' flatbed or bigger. I think that towing with the 3pt may get you into trouble... The lift cam come up and that would put the load over your axles... big time flip hazard.. not to mention, that you are probably approaching the weight limits of that little tractor.. my 1700 had a floating front end when I picked up 5xx# in the rear scoop... I could do wheelies... I added about 200# to the front and all was well... Your 1500 is a bit lighter than my 1700.. not sure if you could safely add enough front weight to it.. Alternately.. if you have a swinging drawbar.. I'd pull from there.. not the 3pt.
Whatever you do, BE SAFE!
Soundguy