I don't know if you feel like going through this much work but: You can make a "fake" trailer with a simple "T" shaped beam with a couple of blocks to act as a fulcrum...then do as this guy did:
The boat stays on the trailer (Or your mock up of a trailer) the entire time. Put the scale under the tongue near the end. You can even put a block on the scale to lift the tongue so you can see the dial. Record the weight. Now, slide the boat aft on the trailer 12" to 18". You must measure exactly how many inches you moved it, and record that. With the scale in the same location under the tongue, record the new scale reading. Finally, measure the distance in inches between the trailer axle (wheel centerline) and the point on the tongue where it touches the scale. You have then four measurements:
W1 = first (heavier) scale reading in lbs, W2 = second scale reading in lbs, X = the distance you shifted the boat in inches, C = distance between trailer axle and scale point in inches.
The formula is Boat Weight = C (W1-W2)/X
A couple of notes. The result, like any measurement, is sensitive to the accuracy of the input data. In this case, the most critical is the distance you slide the boat. One inch error out of 12 inches will really make a difference in the result. Slide the boat as far as you can and still have a readable load on the scales. Notice, too, that by subtracting the two scale readings, any constant error in the scale is canceled out.