Speed to power calc on a 'basic' magnetic trainer
The short answer to your question is: no, there's not enough information. 

In the main, the information you lack is about your particular trainer's resistance curve as it varies with roller speed. Each magnetic trainer has a particular "design curve" for its resistance, and you can see some examples here. As you can see, magnetically-braked trainers tend to have a "flatter" power curve than fluid or turbofan trainers. Different manufacturers will use different numbers of magnets and different thicknesses and materials for their rotor in their resistance units. Some magnetic units allow you to alter the distance to vary the resistance. And, each load generator typically has a way to dissipate heat so the load often varies even for the same roller speed when the unit is cool vs. warm. You can often see this by directing a fan at the load generator and noticing a change in resistance. In addition, as parts wear over time the load units and bearings can become misaligned. 

You are (almost surely) correct that your total mass (you + bike) is irrelevant since for (almost) all basic magnetic trainers, the roller-to-tire contact is set by turning a knob to press one against the other. That roller-to-tire contact is the same whether you are on the bike or not. When you ride on the road, on the other hand, the road-to-tire contact is determined by your mass. As an aside, trainer rollers are typically about 3 to 4 inches in diameter so for a given total mass the roller deforms the tire more than it would on a regular road surface, so a Newton of force on a trainer roller imposes more rolling resistance than the same Newton of force on a road.

BTW, here is a way to check your "cask on pedal" method for calibrating rolling resistance: Adjust the tire-to-roller contact so that your 5.2kg weight just begins to move the rear tire in a 48-14 gear ratio, as you've been doing. Then, vary the gear ratio and find the pedal weight that will once again move the rear wheel. The ratio of mass to gear ratio ought to be constant.