I've always assumed that the base thief ability is the default for any non-thief. My basis is that is how detect noise is addressed in the DMG. In Ch. 15: DM's Miscellany Table 83: Chance to Hear Noise by Race is just the base thief ability modified by the racial modifiers in the PHB. I've always just taken that to suggest that everyone has the base thief abilities, plus racial, armor, and dex adjustments. If you want to keep going from there and open them up to every class, just use the NWP system. The only thing I'd suggest is to consider all of the abilities, except RL solely Rogue and thus it would take 2 slots to get 5%.
It was also only listen that explicitly had the same 1st level thief chances for everyone in 1e and B/X Basic which did not have the discretionary spend percentages for thieves. 1e in the DMG it says everyone has a (really poor) d20 chance based on race which as percentages match up to the thief base skill and racial adjustments and in B/X Basic a 1st level thief starts out with the same hear noise chance that everyone has using the same d6 mechanic which makes it a sort of stealth third level and higher only class power for them in both systems.
So I made the opposite inference. Listen is the only thief skill with with the same mechanic and same starting chance as everyone else so the ones that do not have that power explicitly as a thing that anyone can try seem to be intentional in not doing so. So I inferred things like picking locks and picking pockets are not things that can really be done by the untrained/unpracticed. I IRL certainly would not expect to successfully pick a lock having had no experience even trying to do so.
If you give the 1st level thief chances to everybody you are pretty much giving a 1st level thief zero even crappy unique powers. What is left is the ability to wear leather armor and wield some weapons (limited in AD&D, any in Basic) and backstab. In 2e they get the discretionary points as well as their class ability.
1e DMG page 60:
LISTENING AT DOORS
In addition to the simple exercise of observation, many times characters will
desire to listen, ear pressed to a portal, prior to opening and entering. This
requires a special check, in secret, by you to determine if any sound is heard.
Because of this, continual listening becomes a great bother to the DM. While
ear seekers will tend to discourage some, most players will insist on having their
characters listen at doors at every pretense. First, make certain that you explain
to players that all headgear must be removed in order to listen. Those wearing
helmets will probably have to remove a mail coif and padded cap as well,
don’t forget. The party must also be absolutely silent, and listening will take at
least one round.
Silent creatures — undead, bugbears, etc. — will never be heard. Sleeping or
resting or alerted creatures will not be heard either. If there is something for the
listener to hear behind the door, the following probabilities will determine if any
sound is heard:
Race Of Listener Chance Of Hearing Noise
Dwarf 2 in 20 (10%)
Elf 3 in 20 (15%)
Gnome 4 in 20 (20%)
Half-Elf 2 in 20 (10%)
Halfling 3 in 20 (15%)
Half-Orc 3 in 20 (15%)
Human 2 in 20 (10%)
Keen-eared individuals will gain a bonus of 1 or 2 in 20 (5% or 10%). Use
chance of hearing a noise to determine if a character is keen-eared the first
time he or she listens at a door, and if it is indicated, tell the player to note the
fact for his or her character. Player characters will not initially have hearing
problems (as they wouldn’t have survived if they had them). During the course
of adventuring, great noise might cause hearing loss. Handle this as you see fit.
A loss of hearing might negate the chance to hear something behind a door
without any other noticeable effects.
Hearing Noise: When a die roll indicates a noise has been heard, tell the
player whose character was listening that he or she heard a clink, footstep,
murmuring voices, slithering, laughter, or whatever is appropriate. (Of course,
some of these noises will be magical, e.g., audible glamer spells, not anything
which will be encountered at all!) Be imprecise and give only vague hints; never
say, “You hear ogres,” but “You hear rumbling, voice-like sounds.” Failure to
hear any noise can be due to the fact that nothing which will make noise is
beyond the portal, or it might be due to a bad (for the listener) die roll. Always
roll the die, even if you know nothing can be heard. Always appear
disinterested regardless of the situation.
Maximum Number Of Listeners: Each listener will take up about 2½’ of
space, so up to three can listen at a typical dungeon door.
Maximum Length Of Time For Listening: Only three attempts can be
made before the strain becomes too great. After the third attempt, the listeners
must cease such activity for at least five rounds before returning to listening again.
1e PH page 28:
B/X Basic page B8:
Page B21:
OK seeing the specifics again my memory was wrong, 1st level thieves in B/X are explicitly twice as good as other humans at listening.