Update! Turns out the API already had what you needed – rah and delta – but they were undocumented until now. Thanks again, @insti!
(Implementing roadfunc may still be worthwhile but if you just care about the values of the road today and at the akrasia horizon, the API gives you those directly. Well, almost directly. The road value today is curval + delta. The road value at the akrasia horizon is rah.)