Shielding system would work... burrowed lings splatter blood everywhere... not very shield-like.
I would recogmend having hyper triggers detect the ling's absence, and repeatedly create and kill a large hallusinated unit for a period of time using a timer (unit moving across a piece of terrain).
Scarab Grenade system won't work as far as I know, Scarabs are not controllable in direction after you move them from the point of attack. However, maybe you can move 1 reaver around a target for direction and maybe you might get some angels when it leaves your unit. Not a good idea.
I had an idea a long time ago that I recogmended to someones jail break map... I forget who it was... It was for weapon changing, ammo system, and melee.
Your unit would be a detectable convertant, zergling with burrow, hydra with burrow and lurker evolve, defiler with burrow, siegetank with siege, hero siege tank with siege, mutalisk with guardian and devourerer, hero ling with burrow, hero hydra with burrow, and can be units that use minerals to be detected, reaver with scarab, carrier with intercepters, scv with an air unit to repair above it.
After you picked your detectable convertant with being burrowed, changing form, or using minerals for detection, you can apply this system.
Ammo for enemy virtual HP hits/ kills.
Burrowing, changing form, or using minerals can activate your unit to toggle between weapons your unit has picked up. Each time you toggle, it will move a cloaked unit for the attack effect near your hero using mobile grid to see where your hero's side is. You can use double toggles to activate manual reload, toggle either hand's weapon, use healing pack, etc...
You select the weapon you have, the ammo it has left is displayed with gas or minerals if you choose not you use mineral detection for your detectable convertant and you have the weapon active while in long range battles or medium range battles where your hero is allyed to enemy, but your weapon is not (computer for each human player to detect kills/HP deaths) and if you engage next to an enemy with your hero, your weapon is switched off (sound effect for weapon removal) and you melee your enemy. Your enemy hits virtual HP on your hero which provents your hero from running away. This can be added with Shielding by making the first # of Virtual HP Deaths display hyper triggered events of hallusinations and a sound effect to cover the Zergling's death undernieth you.
This is a good weapon system, but the sacrifice is of not using non-detectable convertant units.
I hope you understand this concept, if applyed correctly, you could have a good system going. You could switch it to non-virutal HP for enemys but ammo would be based off of a kill, meaning all weapons would be single hit kills. Not very realistic unless your hero has tons of life/shielding.
Anyways, good luck on this map, I hope this idea help you with your own ideas.