We have 3 ideas..
#1 A wounded player weighs as much as his max hitpoints, + carried gear.
Or
#2 A wounded player weighs as much as his 1/2 his STR die + carried gear.
Or
Forget weights, and keep it abstract, move wounded is a long action which requires a -2 mobility roll to move 2 hexs, this adds up to -4 if either the wounded or the carrier has a backpack.
Reality: the wounded player weighs more than his STR die. That backpack you carry with a max of 12 encumbrance units weighs less than a person. Although soldiers fully loaded may be carrying in excess of 100 lbs, the soldier himself likely weighs 170. So trying to make rules re what a person can carry is futile; the encumbrance system is not really meant to cover the maximum weight you could conceivably move over a relatively short distance.
I would keep this abstract:
1. you can carry another character but not if either character is wearing a backpack (combat gear is okay);
2. picking up the character is a slow action;
3. moving while carrying a character is a slow action (you can't run);
4. carrying a character requires a Mobility roll;
5. on success you cover additional hexes, per conventional movement rolls;
6. on mishap you fall and suffer a point of damage (the hapless character you carry falls on you and suffers no damage because you are soft and squishy);
7. putting the character down is a slow action.
Basically this is your abstract idea but I suggest you can't fireman's carry someone who's wearing a backpack.
Also:
1. you can drag another character, even if that character is wearing a backpack;
2. dragging a character is a slow action (grabbing them is fast);
3. you can only drag a character 10 m (1 hex);
4. dragging a character requires a Mobility roll, see points 5 and 6 above (on Mishap you tripped).
So this makes it relatively easy to grab someone, kit and all, and drag him into cover, but you can't easily drag him far; to do a fireman's carry is more difficult but you can cover more ground.