If you are using a closed system with a static pressure of 20 PSI and have a pressure running the pump of 25 PSI and assume .49 PSI pre foot of head or 10.2 feet of head can you use the pump performance curve to estimate the flow through the system? Example running the pump with no back pressure should not increase the pressure within the closed system. This is of course impossible due to the friction of the water moving through the system. Therefore the increase in pressure from static is a result of this friction or obstructions to the flow through the system. The pump manufacturer publishes a pump performance curve indicating the GPM of the pump according to the force resisting the pump (Feet of Head). This should give at least a good indication of the flow through the system, similar to the pressure drop across the heat exchanger charts the heat pump manufacturers publish. I am resorting to this because the pressure drop information is unavailable for my unit. Any thoughts?

Yup, you're on the right track - just measure the pressure drop across the pump (ignore the static pressure). Then assuming you have a pump curve it should be about as accurate as doing the same thing with a heat exchanger pressure drop chart. Maybe even a bit better, since the pump will be less affected by viscosity than most heat exchangers. It will certainly get you in the ballpark. Your conversion might be off a bit, though - I believe the standard is 2.31ft of water per psi, so going the other way it would be .43, not .49.

If closed loop has antifreeze density will be different from that of pure water so 2.31 may not apply

Here's a LINK to sort out the gravity influence.
http://www.engineeringtoolbox.com/propylene-glycol-d_363.html

This says if I read it right, 26 F water mixed w/ 10% 60F glycol weighs SG 1.008.This would make the multiplier change not much, to 0.436.

In a closed system gravity has no influence on back pressure only viscosity and friction and obstruction of the flow. If you pump the fluid uphill it must return down hill in the same amount the effort to pump is uphill will be recovered on the downhill similar to, but different then a siphon. Water seeks it own level.

Me thinks the viscosity change would be of more concern than density. But the answer the poster is looking for is yes, using actual data is better than reading from the chart. Why? Because all pumps are not exactly the same and performance varies even with the same model. Thus your pump might be slighly better or slightly worse than the chart shows.