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Material overrides how? With operators?
I don't use Houdini except for support cases. Do you mean this:
https://www.sidefx.com/docs/houdini/shade/overrides.html#overriding-material-parameters-on-primitive...
Actually you'll find it under the Arnold ROP node, attached screenshot.
https://docs.arnoldrenderer.com/display/A5AFHUG/Main (Shader Override)