Couple related suggestions for new maps, or even possibly retrofitting into some areas on existing maps.
First idea: team-aligned respawning howlitzer/polyp/gunpuppies or mini jelly spawners near the team spawn area. Most of the maps are designed with wide or separated spawn entrances to try to keep the enemy team from blockading them inside; this would add another way to keep the enemy pushed back a little. The mini jellies would automatically die and respawn once they left a certain radius of their spawner.
Second idea: control-point aligned respawning turrets or mini jelly spawners. Based on the status of the nearest control point, they'd attack either the opposing team or, if the CP is neutral, both teams. This would help make centrally located control points more defensible.
Third idea: Have a map with a shadow infestation, with some swarm seeds in corners near control points, and maybe some swarm turrets guarding faster, direct routes between areas.
I was going to suggest this some days ago :) My idea was a little different tho. I was thinking about a gun puppy just standing still and then if some knight hit it it would start glowing the other´s team color and attacking your team. I was even going to suggest tortodrones.
But man your idea makes much more sense. I like the second idea more than the others but i think that the whole thing of putting mobs on lockdown would unbalance it. lets say your a striker trying to get a control point but this gun puppies just keep shooting you, that would be a pain in the butt.