I'm suggesting that I'd be happy to have a go at fixing it myself at some stage in the nearish future, if it's not likely to be a big project, but if there were someone else who was keen to do it, I'd be happy to let them! I'm not offering sponsorship.
My thoughts on how such a set of features should work:
1) When editing the nodes of an objectBoundingBox gradient, it remains in objectBoundingBox coordinates in the XML representation, but with no visual change in behaviour.
2) When moving or resizing an object associated with an objectBoundingBox gradient, the gradient's XML representation remains unchanged.
3) We add a toolbar button (or preference or document option) to switch gradients (or a given gradient) between objectBoundingBox and userSpaceOnUse coordinates.
I know C++ well, but don't know Inkscape's codebase. Am I looking at a 2-day project or a 2-week (or 2-month) project?