Yep, we made our own. I haven't heard of PMML before - quite cool! What we've made is a bit more readable for what we're using it for though, IMO. Looks like this:

    {
        "intercept": 1.0,
    
        "features": {
            "feature_1": {
                "coefficient": 1.0,
                "range": [0.1, 10.0],
                "mean_feature_score": 1.0,
                "imputation_value": 1.0
            },
            {
                ....
            }
        }
    }