Man, what if we did spread damage out damage?
-Many tiny bullets: I originally thought of this for autocannons, which can be one big bullet or many smaller ones. What if you roll on the cluster table for the AC and they do that result in damage to one location and the rest of their damage to another location. Bad for something like the AC/20 whose whole deal is one big lump of damage, but it's the concept I'm talking about here.
-2d6x2: The simplest way to determine what locations are hit is to roll twice. A laser might jump from right leg to left arm, but certain angles to allow that. We understand that 'mechs don't freeze every 30 seconds, but are instead moving throughout a turn. Besides, missiles and UACs don't sweat hitting adjacent locations so why bother? It does take two rolls.
-3d6 Adjacency Table: Roll 2D6 and then a third, visually different die. 2D6 gives hit location regularly, then you reference a location-by-location Adjacent Location Table with the 1d6 result and apply the secondary damage there. For example, a Torso Adjacent Location Table might read: 1-2 Arm; 3-4 Center Torso; 5 Leg; 6 Same Location. This is basically what you're suggesting, but it requires 5 new tables for 'mechs.
-Pilot Die: This is how I try to resolve all of my cluster hits, but roll a pilot die, plus a differentiated die for each cluster. Add the pilot die to each cluster die to see where that cluster of damage hits.
-Transfer: The main damage goes to the hit location and the secondary damage location is "outward," the reverse of the damage transfer diagram. If the primary location is the CT, the secondary damage goes to a side torso (which one is determined by ???). Hits to extremities will simply miss, or you could rule they're transferred "in."
-Templates: Lasers scatter in a set 'shape' that's applied in one direction of another. The template is applied to the hit location and the secondary damage is applied according to the template. 'Right' templates will scatter damage from the CT to the RT, or from the LL to the RL, or from the RA to...nothing. Templates could be set by default, but changed with annotation at the beginning of a scenario, depending on the options available.
-Margin of Success: For three and above, it all hits one location (is that one hit or two for the purposes of determining critical hits?). For 0-2, it will scatter, but it's hard to work in a gradual scatter. Maybe you could transfer that bonus to a Torso Adjacent Location Table and construct them such that a MOS of 3 will automatically hit the same location. Or maybe shift damage to the primary location. Or just pass/fail scatter so that damage concentration jumps at a MOS of 3.
I think this works better conceptually for pulse lasers or autocannons, but I get that intent is to take some edge off of the very very good basic lasers. It does mess with the already abysmal IS pulse lasers, but there's no lipstick for those pigs. Sounds interesting. The real trick is doing it without slowing down the game.