Now here's a thought, maybe you could have a spawn rate for the SL. 2 spawns per minute? 4? Then sneaky SLs wouldn't be able to spawn bomb a flag, but you could still keep up a steady firefight. Or you could have people only spawn at SLs at predetermined waves, like one or two minute intervals, so if you fight off the first wave you could kill the SL or face the second wave.
Most radically, you could do away with the SL as mobile spawnpoint entirely, and instead treat the squad as a spawn point, with the a) rate of spawns (spawns per minute=spm) or b) timing of waves denoted by how many people are left alive in the squad. For option A, lets say that for each living member of the squad the squad gains one spawn per minute. If the squad is knocked down to one player, then one minute later the next player will spawn, and thirty seconds later the next player will spawn, and twenty seconds later player four spawns, and fifteen seconds later player 5 spawns, and 12 seconds later the last surviving player spawns, assuming none of the players have been killed in the meantime. For option B, lets say that the timing of the spawn wave is inversely proportional to how many players are alive, with one living squadmate having a wave at 2 minute intervals, 2 squad mates at one minute intervals, 3 at 40 second, 4 at 30 second, 5 at 24 second, and 6 at 20 second intervals. At each spawn wave the entire squad may respawn, so if a team gets knocked down to one person they will be out of action for a while, while a cautious team will be able to maintain itself at nearly full strength. Each player could also act like his own spawnpoint, so if one of them manages to be sneaky the others will have to die before they can join him, one at a time, and infantry battles would be much more dynamic, with squads trying to get players in positions to flank each other or in position to escape and retreat.
For a hypothetical option A', lets say that each player has his own spawn rate of one per minute, and if the team has four players alive, then four separate spawns are reactivated one minute after the last person spawned at that player or one minute after the player respawned. I'm not entirely sure what effect this would have differently than option A, except that you couldn't spawn two players at another player simultaneously and you would consequently restrict the team's mobility.
Just my $.02
Sincerely,
Doc DuFresne