Unfortunately I am so far unable to locate any reference to the option of a "variable Vmcg" which considers reduced engine performance at higher altitudes/temperatures in part 25 or in AC 25-7A. Any idea where else I might look?
AC25-7A contains, in section 23(b)(2)(ii)(B) the following text buried in the paragraph:
If, at the option of the applicant, the AFM value of VMCA is to vary with pressure altitude and temperature, ....
Now, there's no explicit discussion of a similar nature in the VMCG section of AC25-7A, but the actual regulation with respect to engine thrust is essentially the same for both 25.149(b)(1) and (e)(2) i.e.
Maximum available takeoff power or thrust on the [operating] engines
So that would tend to suggest that scheduling thrust with altitude is just as valid for Vmcg. Certainly the regulations do not prohibit doing so.
There is however a further issue that Vmcg is not a manoeuvre that is generally established by analysis, but rather by demonstration. So there might be some arguments about how to establish the Vmcg values at the edges of the alt/temp boundary, without test. Corrections for engine thrust in Vmcg tests are generally rather crude when used to correct test conditions to the "target thrusts". But a similar technique would be possible, and could even be applied conservatively (since even partial credit for the thrust lapse is better than none).