Swords & Wizardry - no. Delving Deeper - no. LotFP - no. Heros Against Darkness - kind of, but no. Secret Fire - uh, if its there it's probably encrypted. Barebones - nope. Crypts & Things - no. Champions of Zed - don't know yet...
Joseph Bloch's Adventures Dark and Deep - A Curious Volume of Forgotten Lore mentions an Insect Swarm spell in passing without giving the actual spell or anything like in the monster section.
Goodman's Games DCC RPG uses the following mechanics
Vermin attractor. The spell attracts a swarm of bothersome insects to the caster’s location that arrive 1d4 rounds after the spell is completed. These insects mill about the caster, biting for 1 point of damage and causing the caster to suffer a -1 penalty to all saving throws, initiative rolls, and spell checks for 1d10 rounds or until he takes adequate measures to disperse or escape the swarm.
Rat swarm: Init +4; Atk swarming bite +1 melee (1 plus disease); AC 9; HD 4d8; MV 40’ or climb 20’; Act special; SP bite all targets within 20’ x 20’ space, half damage from non-area attacks, disease (DC 5 Fort save or additional 1d3 damage); SV Fort +2, Ref +4, Will -2; AL N.Labyrinth Lord uses
Ordinary rats range in size from 6" long to 2' long. They can be black, brown, or gray in color. They live in packs that can consist of great numbers. Rat packs get 1 attack per 10 individuals, inflicting 1d6 hit points of damage per hit. If a character is swarmed by a whole pack, he must succeed in a saving throw versus death or fall to the ground under the writhing rodent horde. He may stand up the following round, but makes a new saving throw if still under the swarm. The opponent can make no attacks until he gets back on his feet.
bats are small and do not attack for significant damage. However, when in a group of 10 or more they are able to confuse by swarming around an opponent. When confused by a swarm of bats, an opponent makes all rolls to hit and all saving throws with a penalty of –2. No spell casting is possible. Normal bats are not particularly prone to fight, and as such they are susceptible to a morale check once each round. This does not apply to bats that are under another's control.I like the distraction mechanic. Blood & Treasure by John Stater has a pretty good treatment
Any creature covered by a swarm must make a Fortitude saving throw each round or be distracted, suffering a ‐2 penalty to all attacks and saving throws. Distracted spell casters must succeed at a Will saving throw to cast spells.He also goes on to outline how you damage a swarm since in his version they are immune to physical attacks
- A lit torch deals 1d3 points of fire damage per hit.
- A weapon with a special ability such as flaming or frost deals its full energy damage with each hit even if the weapon’s normal damage can’t affect the swarm.
- A lit lantern can be used as a thrown weapon against a swarm, dealing 1d4 points of fire damage.
- A spell that does damage to an area does full damage to a swarm.