Well Kitty, I've got something in mind, its not perfect but...
IMHO, usefulness/metal ratio on the units is a good indicator. Basically, the higher is the ratio, the more M you get. now usefulness U is defined as follows:
U = (enemy cost destroyed (how much all enemy units killed by player cost) + metal cost shared + total energy excess * overdrive conversion ratio) / (Metal spent)
Therefore if we define total team usefulness W=sum(U_i), we can calculate the per-player share as follows:
S_i = (U_i + C*W)/(W +N*C*W), where N is the number of players, and C=0.1 (to allow useless players build something at back)
This way energy tycoons get rewarded in the same way as players fighting at front, and reclaiming/sharing is also rewarded. Sitting at back and not doing anything useful will eventually leave you with almost no income at all.
In fact, assuming you deal no damage and OD is not running, N = 4,
when about 3000 metal is destroyed you will get S
0+300/(3000+1200)
0.76 metal income, which is just about what you deserve, while other 3 players (assuming they did 1K usefulness each) will get about S = 0.309 metal income share.
If you excess 50 E/s and OD rate is 110%, you will get about 5 usefulness/second income, which is equivalent to killing a 300 M unit every minute.(I am not certain it is enough though...)
If and when the player's M storage is full, his usefulness is assumed to be zero, so he gets a minimal share only. This is needed to stabilize the system so that the excess metal is not counted as actual usefulness.
Excess metal is distributed using the same system during next frame.