Sorry, meant to share the answer I received explaining the prohibition on FLEX. Despite the answer, I'm not sure I've wrapped my head around it. It was written by an engineer a heck of a lot smarter than I, so hopefully I have it right.
When calculating FLEX data, Vmcg is calculated at the OAT, while Vr is calculated at the assumed temp. As weight decreases, given the differing temps used in the calculations there's a risk that Vr will get closer and closer to Vmcg. Eventually the Vr at the assumed temperature will cross over and go below Vmcg. This meaning (my assumprion) that you could find yourself counter to FAR part 25 by having a rotaation speed lower than Vmcg.
Clear as mud?