This isn't possible in regular 1.19, too, but there is a workaround with mixins. This mod already uses mixins, so I don't see a way to work around that with this mod.
After a little bit of testing, I figured out that an easy fix would be to change this line of code:
vec3f2.lerp(vec3f3, Math.max(0.0F, gamma - darknessFactor)); (line 119 in https://github.com/ItsBlackGear/TheWildBackport-Master/blob/main/common/src/main/java/com/cursedcauldron/wildbackport/core/mixin/client/LightTextureMixin.java) to vec3f2.lerp(vec3f3, Math.max(gamma < 0 ? gamma : 0, gamma - darknessFactor));
This little change wouldn't affect anything by default, but it would help out when you want to set your brightness to -1. I really hope, this will be implemented in a future update.