Thank you very much for the responses gentlemen.
I believe it is very likely the case that the published Vmcg value in the AFM is indeed a "single Vmcg value" established to meet a regulatory requirement to publish such a number and may not necessarily reflect the actual Vmcg under all conditions of weight, altitude and temperature. It seems likely that the published Vmcg value of 115.5 KIAS could have simply been the highest value obtained during testing.
The very fact that a "minimum V1 and Vr" line exists on the chart at all appears to imply that the certification option of establishing a Vmcg value for each condition of WAT which might produce a V1 below the "single-valued" published Vmcg of 115.5 KIAS was exercised. To raise the V1 to a value equal to or greater than 115.5 KIAS would certainly have increased the field length required at higher altitude and temperature field conditions. In all cases where V1 is less than 116 KIAS, it appears to occur at temperature/altitude combinations which would be beyond the 731-5BR engine thrust flat rating alt/temp. And of course, only at lighter weights which would allow for these values to fit within field length and takeoff path requirements. I believe we've arrived at an explanation!
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?
Anyway, thanks again for steering my thinking in the right direction by way of your posts. I knew I could count on you. And I expect that my colleagues will appreciate the input as much as I do. Proper credit will of course be attributed!
Best regards,