This works, but the UI here is confusing I think.

Here, a link icon is used, but link what to what? It links the tool orientation to the scene orientation - this reminds me of the confusing locks we use in the Tool Properties for the paint modes.

I think it's a lot simpler and more straight forward if we just drop the link toggle and just add a single dropdown menu instead, like so:

This way, there's no need for a separate confusing link or lock, and we can actually have text to tell the user that this is the orientation.

It also seems a bit arbitrary that we only have this single option for the transform tools, although that would seem less odd if we add more things here. Eventually we could add 'Drag Action' controls to all the transform tools, and a Tweak toggle too, for selecting and transforming in one motion.

@Brecht Van Lommel (brecht), could you try this patch out with artists in the studio?

To finish this patch off, RNA for custom orientation access will need to be reworked.

Since changes here have a history of having unforeseen down-sides, I'd like to first have this user tested.

@William Reynish (billreynish) I'll match your proposal for the final UI (however not convinced 'Scene' orientation should show as if it's just one of the orientations options - this ignores all other options when set). it might be enough to have a separator under the scene button and grey out other options when set.

@Campbell Barton (campbellbarton), this has been tested & approved by the artists in the studio now.