You can fill it but it can be hard on the hull when sitting on the trailer. A lot of weight with minimal distribution compared to it sitting on a lake as intended. Another option is just put a little water in it and move it around, side to side, front to back. Less stress then, same goal accomplished.
I chose to float mine down the street at a local lake. Sits very high with nothing in it. Only leaks were my fish finder transducer mount holes that I left wide open lol.
The first time I took it after repairing the transom I had no leaks.
But last time I did end up with some water coming out the back when I pulled the plug.
The load on the hull being evenly distributed on the water vs a trailer is a good point.
I think I'll start with only a foot of water and see if there are any drips, and then take it off the trailer and tilt the boat different ways to move the water to different portions.
I have acreage and the ground is soft so it won't hurt the hull.
But I could also see trying to move it around with water in it could cause some rivets to come loose.