Diplomacy is one of the skill checks that makes the least sense in an old-school type game, since it's almost always a function of player interaction. I truly can't think of any situations where diplomacy, in particular, makes sense to randomize or to base on a character sheet number unless you're working with an entire suite of skill sets in character generation, and even then I think it's a bit weak as a concept. How to convert or use this concept, focusing on concrete ways in which the challenge can be met through player interaction (examples of concrete methods italicized):
"The guard can be persuaded if he is given a token of authority of some kind, and he is not likely to recognize a forgery."
"The merchant can be convinced to assist the characters if they manage to play on the fact that his business will be badly affected if the dragons keep eating people."
"The bartender is lonely and bitter about his life: any friendly approach will result in a flood of useful information and offered assistance."
This skill in the new-school arsenal presumes that the device has been found already, so it is a direct analogue to the thief skills such as "delicate tasks" (in S&W) or "find/remove traps." What's different is that (a) this skill broadens the ability to non-thief characters, which isn't unreasonable as long as the probabilities are very different for a thief vs. another character class. After all, this is where thieves are supposed to shine. (b) The OTHER issue isn't something structural about new-school approaches, but it's a very, very, very common approach in modern adventure design. In more modern adventure design, the traps tend to get harder to detect as the character level advances. I think this tends to devalue the thief if it's across the board. There should still be plenty of traps and locks that are NORMAL in difficulty so the thief can excel. Granted, the super-awesome traps set by deadlier villains will create situations where there's a really tough challenge, but I prefer to have a range that is weighted toward the idea that on normal, everyday trap-triggers, the thief's advancing levels actually mean more successes. There are other reasons for this, and not space to into it.
So, examples of non-thieves disabling traps and other devices:
(1) is there a reason for not just keeping this in the thief's domain? "A thief can disable this trap normally. If a PLAYER describes the disarming process, it can also be disarmed." [this allows for player skill to trump the character sheet, with no character-sheet option for solving it if you aren't the thief you chose to have for precisely this reason].
(2) "Thieves can disable the trap [either normally or with a penalty or bonus. Members of other character classes can also attempt to disarm the trap, but with only a [base chance, why not, it's got nothing to do with level since it's not trained for their class]. [another alternative, if it's just a test of not having your hands shake, then use a dexterity check] DON'T FORGET to compare the probabilities and make sure the thief has the better chance!!
That's it for this installment -- hope you enjoyed!