Does values greater than 255 even happen with gl_modulate 1 or does lightmap values stay below 255 in that case? I suspect the latter. (Does q2 even have map lighting overbrights like q1 and q3?)
Most likely the clamping you believe is happening is baked into the lightmaps in the remaster .bsps, and not the engine doing it.
If you stuff the original vanilla .bsps into the remaster quake 2 pak0.pak it looks on par with vanilla q2 ref_gl with gamma 1.0, intensity 2 and gl_modulate 1 (minus textures being washed out because it doesn't do intensity like vanilla's way). I welcome you to challenge this assertion; name the location and bsp.
q2vanilla_base1_in_ref_gl.png
q2vanilla_base1_in_q2_remaster.jpg
The conclusion is that the engine is not doing wrong, it is the lightmaps in the remaster .bsps that contain the difference.
Overall the bsps in remaster q2 are much brighter than the bsps in vanilla q2; most likely this is an intentional change in the remaster.
It is agreed by the devs that outdoor areas in the q2 remaster bsp's look like shit and are blown out to mostly uniform lighting. The devs are rebuilding the lightmaps for the next patch to rectify this.
EDIT: I also don't see what you mean with the q1 remaster overbrights. Here is a comparison between Winquake and Q1R looking at the torch bowl on the right of start.bsp from original id1. (brightness '0' in Q1R, and gamma 1.0 in winquake). If brightmaps needed to be * 2, it would become tremendously too bright.
winquake_start.png
q1r_start.png
Just to fully prove that here is a notorious overbright map example from kjsp1.bsp...running in Winquake vs Q1R:
k1jsp_comparison.png
I don't know why you have the impression that lightmaps in Q1R are being displayed at half their intended brightness. The kjsp1.bsp example especially would look much different if the engine was treating the lightmaps as you claim.
I think maybe you are being thrown off by all of the id1 maps (and q2 maps) being relit by more modern tools. This means more realistic light bouncing and propagation and so forth. This does mean some areas are darker, but at the same time other areas are also lighter. In my opinion the relit q1 id1 maps are not darker than the original id1 maps; maybe some corners are but overall in my subjective view it is about the same. Q2 remaster's relit maps certainly are much brighter than the originals. And in q2's case the light sources from the sky currently have tremendously too high brightness values which is being addressed.
Regardless of the subjective views, I think I have proven objectively that it is nothing the engine is doing wrong.