Hi Liam,
do you still want to use the ML model mapping functionality or rather do something custom with the ONNX file in Java? If it's the latter, you might use a FileDocument as the input for the Java Action, then you just need to upload the file somehow in your app.
Alternatively, you can move the ONNX file into your projects folder (for example like this: \deployment\model\resources\ML_Model) which you may be able to access from Java.
I have never done the above, but maybe those pointer help you.
Best regards
another Liam