Having a iframe or embedded "Python Visualization" in Spotfire is a good idea. There are a lot of Python based visualization mechanism exist which follow a simple pattern -
DataFrame ➡️Some Transformation using Python ➡️ Visualization as Plot
Benefit of adding as an independent visualization are -
Easy to duplicate, move and configure visualization
Using python native visuals as details visualization
Low learning curve for existing python audience, to attract more developers
Reusable Python code - to replicate charts as and when needed, without much python experience as well.
Suggested options available may be :
Configure DataTable from Visualization Properties (will act as input DataFrame)
Axes Expression replaced by Python Code Snippet
Allow saving Visualization Code to Library, making it reusable.
I agree that Visualization Mods are awesome, however to create them from scratch needs a learning curve.