I was wondering if a viable LAM could be built at 20 tons, and it appears the answer is yes... as long as your expectations aren't too high:
Mayfly LAM MYF-1Y
Mass: 20 tons
Tech Base: Inner Sphere
Chassis Config: Biped
Rules Level: Experimental Tech
Era: Succession Wars
Tech Rating/Era Availability: D/X-E-D-D
Production Year: 3025
Dry Cost: 2,179,556 C-Bills
Total Cost: 2,179,556 C-Bills
Battle Value: ?
Chassis: Unknown Standard
Power Plant: Unknown 100 Fusion Engine
Walking Speed: 54.0 km/h
Maximum Speed: 86.4 km/h
Cruise Speed: 162.0 km/h
Flanking Speed: 248.4 km/h
Thrust: 5
Overthrust: 8
Jump Jets: Unknown
Jump Capacity: 150 meters
Armor: Unknown Standard Armor
Armament:
1 Medium Laser
1 Recon Camera
1 Remote Sensor Dispenser
1 LAM Conversion Gear
Manufacturer: Unknown
Primary Factory: Unknown
Communications System: Unknown
Targeting and Tracking System: Unknown
================================================================================
Equipment Type Rating Mass
--------------------------------------------------------------------------------
Internal Structure: Standard 33 points 2.000
Engine: Fusion Engine 100 3.000
Walking MP: 5
Running MP: 8
Jumping MP: 5 Standard
Jump Jet Locations: 1 CT, 2 LT, 2 RT 2.500
Heat Sinks: Single Heat Sink 10 0.000
Heat Sink Locations: 1 LT, 1 RT, 2 LL, 2 RL
Gyro: Standard 1.000
Cockpit: Standard 3.000
Actuators: L: SH+UA+LA+H R: SH+UA+LA+H
LAM Conversion Gear: 2.000
Armor: Standard Armor AV - 69 4.500
Internal Armor
Structure Factor
Head 3 9
Center Torso 6 10
Center Torso (rear) 2
L/R Torso 5 8
L/R Torso (rear) 2
L/R Arm 3 6
L/R Leg 4 8
================================================================================
Equipment Location Heat Critical Mass
--------------------------------------------------------------------------------
Medium Laser RA 3 1 1.000
Remote Sensor Dispenser RT 0 1 0.500
Landing Gear RT 0 1 0.000
Avioncs RT 0 1 0.000
Recon Camera LT 0 1 0.500
Landing Gear LT 0 1 0.000
Avioncs LT 0 1 0.000
Landing Gear CT 0 1 0.000
Avioncs HD 0 1 0.000
Free Critical Slots: 29
BattleForce Statistics
MV S (+0) M (+2) L (+4) E (+6) Wt. Ov Armor: 2 Points: 4
5j 1 1 0 0 1 0 Structure: 2
Special Abilities: ENE, SRCH, ES, SEAL, SOA