@melanol I'd much rather be the new player assigned to the frontline position and told to fight than be assigned to a rear type position and left to do what I want.
I very nearly quit FAF for just that reason as a new player - I joined some setons games and did well at mid (even getting a kill against my opposite number before quickly overextending mu acu and dying in my first game); then with c.10 games to my name I joined a lobby where my team thought it would be a great idea to ignore me saying I had 0 experience as an air player and stick me on the air slot instead of mid. End result was me being flamed for the enemy team's experienced air player crushing our team.
As to the post suggestion, you can achieve a lot of the desired effect with a really low unit cap that doesn't get shared on death (which I think is an option? unless it's changed), although I suppose a mod could refine it to make it a bigger incentive to keep teammates alive (since the unit cap treats 1 mantis as equal to 1 megalith).
Personally I wouldn't want to play with it, but I could see how it might appeal to some.
As to whether it's possible, I don't know if it's possible to adjust how the unit cap works directly to make some units count as 'extra' units, but if not a workaround could be to adjust an individual player's unit cap maximum so it is decreased where a T2+ unit is built which would have the same effect.