I don't remember anyone requesting this particular feature, but it makes perfect sense. What about the UI, though? (I cannot try your patch right now, as I'm in the middle of an upgrade and cannot yet compile.) I propose the following: pressing H and V in the Node tool already flips the selected nodes; all you need to do is to check if any of the selected nodes is mouseovered and if so, use it as the center. This way you can flip only the selected nodes or the entire path if you select all nodes. It does not require any new menu commands and is very easy to do; also it will be consistent with other similar actions - for example, when you join two nodes, usually both go half-way to join, but if you mouseover one of the nodes, it stays put and the other one jumps up to it.
Also in Selector, there's such thing as the rotation axis that can be freely dragged in rotate mode (and it's remembered and saved for each object). The selector's flip functions (also called by H and V) do not use it at the moment for flipping, but I think they should.
This would have been my suggestion... use the rotation centre for flipping and have it snap to nodes with some short cut (or even standard and have a shortcut overrule it, since it already snaps to corners etc, which I consider to be great). It's just the most logical thing, I can think of right now.
Thanks!
David