gave it a try. on basic materials Localized IBL works fine in Forward rendering in 4.25.
since you mention “material nodes/functions being used that are not available in forward” I’m getting suspicious. my IBL engine changes are not using any material nodes/functions -at all-, it’s all engine code and engine shader changes.
it might be your materials not being built for deferred (i.e. it wouldn’t work on a stock precompiled UE4 build either). or maybe somehow my changes are conflicting with some of the more advanced material functions (those that rely on custom HLSL-code nodes).
to be sure I’ll have to test the same material in a precompiled UE4 build.
a couple of screenshots with Localized IBL in Forward renderer.
default map with a half roof and a green floor propagating green IBL:
seaside town from marketplace (which has many materials of its own). this had a compilation error in the Landscape material, which was fixed when I removed the ParallaxOcclusionMapping node