I can see this working. It’s a combination of my proposal and what @Langlaeufer proposed further up in this thread.
On footway, oneway=yes by itself is unclear, but oneway=yes oneway:foot=no is clear, as is oneway=yes oneway:foot=yes.
With the proposal approved, there will be two ways of tagging oneway on footway unambiguously. One is avoiding bare oneway= and just tagging the mode-specific one-way tags, e.g. oneway:foot= and oneway:bicycle=. The other is generic oneway= plus oneway:foot=.
The implications of both tagging options for editors and data consumers are the same: bare oneway without oneway:foot should be flagged by validators. Presets should discourage mappers from setting just oneway=. (Only talking about footways here.) Pedestrian routers and pedestrian maps only need to look at oneway:foot.