Nothing official. (I don't know if there are any, but there's a d20 setting with wild west elements, if I recall correctly).
Should be like this:
- Exotic Weapon (with higher-than-normal penalties if you don't have the proficiency)
- Make a trouch attack. If you hit, the lasso's around the enemie's body (or a portion thereof). You then can make trip attacks (see whip for details), If you drop the lasso, you can pick it up again as it's still around the enemy.
- You get +2 on disarm checks, but only on weapons that do no slashing damage (as they would just cut through).
- damage: none (but see below). Crit: 17-20. If you score a crit when trying to get hold with the lasso, you hit the neck and can deal 1d6 strangling damage per round. If you manage to get the lasso 'round a branch or something and pull the enemy up, he'd also start to suffocate
- to get free from a lasso, you must make an escape artist check (dc = your ranged attack bonus, as you can hold it taut), a strength check (against your str check) or cut it with a slashing weapon (standard action, attack against AC 10 or so needed)
I just made that up as I went along, but surely others will contribute to this, and I will improve one thing or two. For improvements, you can look up the whip (in PHB), the garrote (in Song & Silence) and Use Rope and Escape Artist Skills.