Second thoughts:
1) The alternative connection that I described would not result in _any_ parallel path current flow. In the case that the three-way switches were 'on' then the 'shunting switch' would result in the other traveller leg being energized, but no current flow through this alternate path. In the case of the three-way switches being 'off', the current would flow through the shunting loop and one of the traveller legs, but again not the other.
2) The first paragraph of 310.4 ends with '....shall be permitted to be connected in parallel (electrically joined at both ends to form a single conductor).' As I now read 310.4, it does not seem to be a general prohibition against parallel paths for the flow of current, but instead a set of requirements for treating multiple separate conductors as a single conductor for the purpose of determining ampacity and OCPD rating. If you do not meet 310.4, then you can still have parallel paths, but each path individually must comply with code.
-Jon
