cwatters,
I think there is still a problem in your final re arrangement of the formula.
It still contains two unknowns- Drag and the incremental increase in drag with a towed object.
I suspect it could be solved using a differential equation however my maths is not good enough to go down this path.
Changing the approach to something I read on the calculation of drag on a car you could:
1 set up level flight at some speed of interest
2 Quickly and accurately set zero thrust. Keep the aeroplane in level flight
3 Time the change in speed over a small speed range to compute acceleration.
4 Calculate Drag Force from F= m a
The problems with this are that it assumes deceleration is linear (which it is not ) and accurately setting zero thrust.
Greedy