I seem to recall that Bottlehead had the necessary switching jack available to upgrade the standard one while continuing to use the IHF-standard resistors. I certainly can't seem to find it now, though.
Perhaps an e-note to Bottlehead could tell you if I'm losing it, or such a thing actually exists. IIRC, they sold it with a couple of new resistors to aid in the retro-fitting, since the existing resistors would be difficult to re-use.
Alternatively, I believe a double pole switch would be all you would need to make a "Headphone OR Speaker" arrangement; you would switch the left and right "+" output signals to go to either the speaker outputs OR the headphone jack resistors. As far as I know, there would be no reason to switch the "-" output lines, they could stay connected.