I'd be inclined to try a very tight yellow flame to the center (only) of the Allen bolt for maybe 30-40 seconds, then flip the bike and drench in a penetrating fluid (after during off flame!). Nowhere near red hot, just 500 or so degrees to get the heat down the bolt. Also tap lightly after heating, several swift taps though not hard. Do this several times over a week. Aluminum is a great heat sink and I doubt the stem will be bothered but you can wrap it and the head tube in ice. Temperature differentials can often free parts. The absolute worst corrission situation is high carbon steel threads and aluminum (weren't those head tube slip jam fittings aluminum? Can't remember) and you might be there. Gentle force might work over time given focused heat, light shock, penetrating fluids.