When measuring input current to a primary winding of a transformer with an inline true RMS multimeter, can no-load current be measured, then full load current be measured, and then simply subtract no-load current from full load current to leave the apparent current being used for load and any losses?
Measuring the load and no-load input power of a transformer gives you a reasonable idea load power, but it's not strictly accurate. The losses in the transformer are not constant and will depend on what the load does, even with a fixed input voltage.
The best way to measure load power is to measure load power. Put the RMS meter on the secondary of the transformer to measure it directly. If you are trying to measure losses in the transformer, then you want to measure both input and output power. The output power will be less, and the difference will be the losses in the transformer.
Another less invasive but more elaborate way to measure the losses would be with calorimetry. Just about all the power lost will turn into heat. If you measure this heat power, you will know quite accurately how much the transformer is losing. If you also measure input power, then you can calculate output power. Note however that the calorimetry method of measuring loss power is inherently very low pass filtered. Put another way, it will take seconds to minutes for the reading to catch up to a new steady state.
No-load current is mostly out of phase with the voltage, so if you just measure RMS current into the primary, you're going to see quite a bit of power there...most of it imaginary. The imaginary component isn't a loss, and it won't heat up the transformer. You'll need something beyond a simple multimeter to find out what you want.