Are their chem weapons or artillery that are more mech scale?
Interstellar Operations has rules for chemical weapons. They tend to go heavily to "GM/player judgment" territory though, since delivery methods are up to players.
Like, players can rule that an Arrow IV (which can be carried by 'Mechs) has missiles with chemical warheads. Becomes very complex fast though.
IO includes other WMDs as well, with nuclear weapons having most complete rules.
In practice, those don't really work in normal games, being too complex and too powerful. If you want to spice your game with special weapons, normal artillery and air strikes are probably good bets. BattleMech Manual has "support powers" for simple artillery and air strikes (and countering those), and one could modify them a bit with rules from Tactical Operations for inferno warheads or the like. Fire is simpler, yet fits the Word of Blake doctrine well enough.
Some 'Mechs can carry Arrow IV artillery missiles, but i can't off hand name any the Word of Blake has except the very limited Catapult C3 which has only one ton of missiles (C5 is better but pretty much Capellan-exclusive, though the Word of Blake could always have stolen some, or bought few, whatever works on your table).