I've got photos of my masthead crane somewhere, but the simplest way would probably be to replace the masthead plate (the bit covering the open end of the extrusion) with a bigger bit of AL. one that overhangs the front of the mast with enough material to drill a hole for a swivel in it. Keep the old top-plate to put back. You will also want to probably up the size of the screws holding the plate in place to accomodate the sheer -but none of that should take you out of compliance.
That said, I agree that you might want to make the plate "oversize" on the sides as well and rig up some spectra removable runners to take the lateral loads on the mast.. I'd attach them by quick release shackle to the stanchion plates and only have them on when you raise the kite.
Something like turn down deep, set kite, connect runners, cinch up windward runner (4:1 should be fine) ease the backstay and then heat up.
for the gybe, back stay on, turn down blow windward runner, gybe, cinch up weather runner, backstay off.
But I'm not convinced the area you get will make up for the ratings hit