The truncheon sounds like what I was looking for. Looking at the Book of Exalted Deeds, 12lb seems ridiculously heavy (not an issue for the character in terms of encumbrance, but it is silly) for the truncheon, though the stats look about right.
Of course, the downside is that you can use Weapon Finesse with a Sap but not a Truncheon. Rather interesting trade-off, though somewhat esoteric. The character is a high-level Fighter/Rogue/Blah with Shortsword: Focus, Improved Critical, Specialization and Melee Weapon Mastery using a Sunblade +3 (i.e. one better than normal) most of the time. The comparison would be something like this, as the character has 5d6 sneak attack.
For two-handed use:
1. Masterwork Truncheon: attacks +26/+21/+16/+11, damage d8+9+5d6 = 31.0 damage/hit.
2. Sunblade: non-lethal attacks vs. non-evil +32/+27/+22/+17 (that is with the -4 for non-lethal factored in), damage d10+16 without power attack = 21.5 damage/hit.
3. Power attacking for the difference in attack score, the Sunblade is: non-lethal attacks vs. non-evil +26/+21/+16/+11, damage d10+32 = 37.5 damage/hit.
4. Masterwork Sap: attacks +31/+26/+21/+16, damage d6+9+5d6 = 30.0 damage/hit.
For single-handed use:
1. Masterwork Truncheon: attacks +26/+21/+16/+11, damage d8+6+5d6 = 28.0 damage/hit.
2. Sunblade: non-lethal attacks vs. non-evil +32/+27/+22/+17 (that is with the -4 for non-lethal factored in), damage d10+13 without power attack = 18.5 damage/hit.
3. Power attacking for the difference in attack score, the Sunblade is: non-lethal attacks vs. non-evil +26/+21/+16/+11, damage d10+21 = 26.5 damage/hit.
4. Masterwork Sap: attacks +31/+26/+21/+16, damage d6+6+5d6 = 27.0 damage/hit.
So the sap is probably the best one-handed option for non-lethal, and using the Sunblade to do non-lethal is the best two-handed option for non-lethal. Only problem of course is that the Sunblade glows brightly - kind of loses the element of surprise!