I’ve created a pull request for this feature:
https://github.com/EpicGames/UnrealEngine/pull/6646
It adds a GetDistanceAtSplineInputKey GetDistanceAlongSplineAtSplineInputKey method to USplineComponent.
The change relies on the ReparamTable assuming a linear relationship between the distance and the input key between consecutive entries. Meaning if distance is 5 at input key 0.1, and distance is 25 at input key 0.2, then distance at input key 0.15 is 15.
So this should be as accurate as the inverse operation(s) (e.g. GetInputKeyAtDistanceAlongSpline, GetLocationAtDistanceAlongSpline), without the need to iterate.