Hi Malc,
I know this is beyond my equipment and skill. But maybe not beyond the capabilities of the very best amateurs with serious equipment.
I worked with Bruce Gary on the White Dwarf Exoplanet survey in 2011. I could achieve 5 mmag precision on a 13 Mag target. A 15 Mag target is more like 50 - 100 mmag scatter for me. This is with a C11 and Atik 320E CCD.
The guys at Mt John in NZ got < 1 mmag on the same target using a 24 inch scope + Apogee Alta U47 camera. So a setup like that in similar hands might just do it for a 15 Mag target.
There is some interesting info, including a list of skilled observers who participated, at the PAWM website
http://brucegary.net/WDE/. There were many N.H. guys involved.
-Ivan