Did you consider making a separate filter for this new feature? That is probably simpler than modifying each of the other filters to accommodate this new feature. You would only need to write a new model function (registered with model=True). You wouldn't need to write a new simulation filter, you would only need to add a registration on top of existing filters (e.g., analogous to how ivar_basic and raw_ivar_basic share a simulation filters). Maybe there is a downside to this but I think it would be more modular/easier to maintain.

If you prefer to modify the existing filters, I think you missed adding your new feature to iso_harmonic_ivar_scaledep_model