I'm not a measurer, but I think you will find that under the ERS the pole you are referring to is classified as a Jockey Pole, not a Whisker Pole. Whether or not it is allowed will be determined by the relevant class rules.

This is not a whisker pole you speak of.  This is a spinnaker pole, and yes, asymmetrical spinnakers can be squared back just as a symmetrical spinnaker is - that's what happens with the tack of the symmetrical spinnaker when you pole back to run deeper.  The symmetrical spinnaker just has a 'reversable' tack and clew where the asymmetrical does not.
Lots of boats do this but the rating agency must be made aware of it if this is new capability.  If the boat was never equipped with a pole to square back the spinnaker the boat's rating will likely get an adjustment.

Per ERS definitions, a spinnaker (G.1.3(f)) is not a headsail (G.1.3(d)). Difference is whether the measurement between the half luff point and half leech point is more than or less than 75% of the foot length. So if the sail ranks as a spinnaker rule 54 would not apply.

I think the typical arrangement is that the aso spinnaker remains tacked to the bow (or sprit, if equipped) by a tackline which acts as a foreguy, and is also connected to an afterguy that runs through the outward end of the pole.

And all sorts of poles and things are also precisely defined in the ERS

(iv) OUTRIGGER A hull spar extending transversely connected to a sheet. 
(d) OTHER SPARS
Other spar types include their rigging, but not running rigging.
Other Spar Types:
(i) SPINNAKER POLE A spar attached to the mast spar and connected to a spinnaker guy.
(ii) WHISKER POLE A spar attached to the mast spar and connected to a headsail clew.
...
(vii) JOCKEY POLE A spar attached to the hull or mast spar, extending transversely and connected to a spinnaker guy. 

Note that these definitions rely on what is attached to the pole, sheet or guy or headsail clew, at any one time, not what the pole might otherwise be used for.  So a spinnaker pole connected to a headsail clew is a whisker pole.