Looking through portals from Immersive Portals seems to register to the crosshair as looking at air, and the crosshair doesn't change based on what you're looking at through the portal

Oh, that is an intriguing compatibility challenge.

I finally got around to finishing this. There is now full support for Immersive Portals, with everything working perfectly fine across dimension boundaries! :D

EDIT: To clarify, the compatibility layer lives in DynamicCrosshairCompat, so that mod needs to be installed as well.