I don't think there is any direct method to get this information. You can try following:
Add 'Unfold' stage from 'Sheet Metal' tab as per the screenshot above. This will give you reference edges where the bend radius starts. You can then create a sketch using 'Convert Entities' to make sketch lines from these reference edges. The bend line will be in the centre of these reference edges. Hope this helps.
You have asked this question in 'API/Macro' section, you should move this into 'Sheet Metal' section.
Formula for length of bend is L = R * Ang where L is the bend length, R equals the bend radius plus the thickness * K-factor, and Ang equals the bend angle in radians. Hope this is what you are looking for?
Sorry I miss understood your question, I thought you wanted to draw reference lines as you have indicated.