[David]

I’ve seen it happen a couple of times that a left handed thrower picks up an out of bounds disc, and plants his left (non-pivot) foot on the playing field next to the line, and his right (pivot foot) half a yard into the field. For the opposition this looks normal (they think the pivot foot is properly set) until the thrower pivots, revealing he has set his pivot foot too far in-field.

After calling a travel, what happens next? Is the thrower required to use his left foot as pivot foot (non-natural pivot foot) as he already established his pivot point there?

Pivot point: The point on the playing field where the thrower is required to establish a pivot after a turnover, or where a pivot has already been established.

[Benji Heywood]

No. After the travel call – which is of course valid in this situation – the thrower may establish a pivot with any part of the body just as normal.

The definition you quote defines the point at which the pivot should be established, but does not determine which part of the body should be placed there. All the relevant rules state ‘establish a pivot’ and not ‘re-establish THE pivot’ – there is nothing to suggest the thrower cannot use either leg.