ok, after much fiddling about getting the right binary format for the edid to load I finally managed to make it work, though a combination of the online tool posted earlier and hex fiend on a mac.
I managed to replace the EDID VSVDB from my TV with one from a newly created edid file, and can at least see the higher luminance being picked up in this log:
DOLBY: v 1: 3840x2160 0->0(T:1-1500),g 1: 1920x1080 1->15000000,vpr
Previously my LG E8 TV was min 1 and max 700 and then changed the EDID VSVDB to:
Vendor-Specific Video Data Block (Dolby), OUI 00-D0-46:
Version: 1 (12 bytes)
Supports 2160p60
DM Version: 4.x
Colorimetry: P3-D65
Low Latency: Standard + Low Latency
Target Min Luminance: 0.00000000 cd/m^2
Target Max Luminance: 1500 cd/m^2
Unique Rx, Ry: 0.70703125, 0.29296875
Unique Gx, Gy: 0.17187500, 0.79687500
Unique Bx, By: 0.13281250, 0.04687500
And get min 1 (code does not allow 0 so bumps up to 1) and max 1500.
So looking good so far, next is to add the VSVDB to a non DV TV EDID and see what that gives!
Thats a bit more involved for me moving around my setup, will give it a go later on.