Technically I think you have to block low first but it will be really difficult since it is a meaty unblockable and 1F blocking would be required most likely. Here is how I understand the setup, which may be false:
After breaking an airthrow in marvel 3 you are invincible to all attacks for a certain amount of frames UNLESS you perform an action (attacking or movement). Because of this, on the way to the ground after the airthrow break the opponent will not get hit out of the air by the slime (or j.M) allowing the slime to hit low and enabling the unblockable setup. Likewise, the slime will hit immediately upon them reaching the ground which is why you cannot hard tag out of this setup and why I think you would have to block low first.
If this is true then the setup will always go like this:
-If they simply try to block, you will get the unblockable. It even works on shorties like wolvie and morrigan that can normal crouch airdash j.m (justice)
-If they try to mash out, with the right normals they can trade with MODOK and you will not get a combo. Or you will hit them airborne super low to the ground which can be difficult to confirm off of.
-Use invincible air supers and fast air ok DPs like chun-li’s to escape.
-Use assists after the tech to mess with the spacing or put hitboxes on the screen to threaten modok.
To eliminate all possibility of escape except for invincible air supers adding an assist will control all of the other options. For example, with DOK/Dorm/Doom you can call dorm+cr.m, airdash, airthrow and if they break the following will happen:
-opponent is invincible to the assist attack so they will still have to land into the slime unblock and cant try to pushblock to stay airborne
-If they mash on attack, they become vulnerable and will get hit by the dark hole before reaching MODOK
-If they call an assist it walks into the dark hole for a happy bday (havent tested them all)
-Be prepared to do your MODOK/Doom bnb after they get hit which will kill everyone.
This works with many assists but try to find some with lots of active frames like eye of agamotto, dark hole, beams, etc that last long enough to catch their mash attempts and assist attempts while being easy to hit confirm off of. With my DDD team doing my typical j.L bnb pre-airthrow I do ~400-500k depending on assists and with the dok/doom follow-up its like 600k before super. With a HBR ender the whole sequence does 1.2-1.3m and you can DHC into ball super(combos if they survived somehow) and tag modok back in for welcome mixups with the ball super or charge spells with dorm and still have a net gain of 1 meter and 1-2 cubes. Please use this technique.
Disclaimer: I dont test stuff on the midgets/ammy so I am not sure what will work on them…fuck rocket raccon.