It is possible in your opinion to blend some rectangular texture and obtaining a not clamped [0 - 1] output texture?
With GL3 onward I think ClampColor is set to FIXED_ONLY by default. Otherwise it would seem you’re constrained by the range of the target format.
In GL<3 context I’m using RGBA_16F rectangular buffer and blending all light contributions into. AFAIK, using FP buffer you’ll not have color clamping.