Oddly, the core idea behind the OP's suggestion does actually exist in BT canon.
The 'Mech-mounted
Fluid Gun (described on page 313 of TacOps, and currently (technically) available in the timeline) can fire, among other things, a cooling fluid described (described on page 361 of TacOps) as "a cryogenic, fast-cooling chemical mix less useful for firefighting than for rapidly reducing the surface temperature of superheated metals" and having TT gameplay effects of "[reducing] the heat level for any heat-tracking target [such as a BattleMech] by 3 points per hit (to a maximum of 9 cooling points per turn)".
It should be noted, however, that it still uses ammo in the standard way (fluid is carried in one-ton tanks, one tank = 20 "shots"), and is still subject to ammo explosions; "Heat-tracking units [such as BattleMechs] carrying coolant ammo will not set off the coolant from overheating, but coolant ammo that explodes from a critical hit reduces the heat level of such units by 3 points, in addition to the 2 points of internal damage from exploding Fluid Gun ammo".
However... while the OP isn't
totally off his rocker, I would highly doubt that the Devs would be in any great hurry to implement the Fluid Gun with this capability...