For those looking for a Feat for high Comeliness, we can base it off of the 'Alluring' feat from WoT d20, which gives +3 to Charisma-based checks against members of the opposite sex. It was a Regional Feat, so let's scale it back to +2. To bring it into D&D, which has multiple races, let's also make it only work on creatures with a race (and sexual orientation) that would generally be romantically inclined towards the character. This feat is significantly weaker than the vastly powerful Nymph's Kiss because it has become more situational (at best, if the DM plays a world with only one race, you can use this against half of the NPCs, unless of course the DM tends to produce gender-bias in their NPC ratio) and it doesn't give the +1 to saves or the extra skill point. Compared to the +2/+2 feats, this gives a bonus to 5 skills instead of 2, but less of the time. I would rule that part of the bonus comes from the confidence, attitude, and wiles garnered from years of growing up coddled as the handsome-boy/pretty-girl, so a player couldn't just gain this by using a Hat of Disguise to turn pretty.
For the flip-side, if you use the 3.x Unearthed Arcana's "flaws" system, I can imagine a flaw called 'Hideous' that gives you a -4 to interaction checks. However, I would be leery to allow this because "Hulk Smash!" players might use it to take a roleplaying hit so they can get more combat feats. Perhaps it needs to be more potent, something like "Your hideous visage causes all NPCs you encounter to initially treat you as one category worse on the reaction chart [so from Friendly to Indifferent for instance]. You may still attempt to influence them higher normally, with a -2 penalty to the Diplomacy check." That might more accurately depict the large initial reaction to ugliness that can be overcome by a charismatic-yet-ugly character, but someone with a Hat of Disguise could still effectively negate this. The upside is that initial reactions are more noticable for a barbarian who never parleys anyway (for instance, when the surly barkeep in the tavern his friends have been raving about takes one look at him and calls the bouncers to throw the hideous abomination out without giving him a chance to speak).
Tell me what you think of this solution. Too powerful? Not enough? A stupid idea to begin with?