I think I found a workaround, sharing it here for anyone else with the same question. I created a copy of the material function MX_StandardSurface that I then used that new material function in Project Settings > Interchange MaterialX.
For the custom MX_StandardSurface I inserted an inverse gamma into the base color input (and for any other inputs you want to use sRGB) like this:
I had to build the 0.455 gamma as shown because, AFAIK, there is no gamma, and the sRGB to Linear and linear to sRGB were giving odd results.