Well, you can see, then, why I was looking more at the broader, more granular heat scale as the basis.
In fact, the Movement mode modifier was a hold-over from that. Maybe instead of a multiplier, the movement mode is a simple one time addition, setting the base activity level.
So, for example, a Phoenix Hawk that runs in a turn will have it's activity level increased to level 3, while one that jumps increases to level 4, and then, any further action simply adds one to the level as the activity happens.
I think this system works only in a modified turn structure. Battletech is played in 5-15 turns normally, so adding a reaction phase like this cuts your turn time to 3-7 turns in the same amount of game time. I am thinking like the solaris game here, with turns having multiple steps of action and reaction, with higher weapon rates of fire and such. It works in that system a bit better cause the number of units is very low, so taking so much more time with 1 unit is more acceptable.
The point of the reaction system also seems to be to limit how much a pilot can do, but this doesnt scale like normal. If you are familiar with HBS Battletech video game, they have a similiar system where every time you are fired upon, you lose one point of evasion (but you generate a bit more evasion then normal). This works cause you have 4 total units. If you have 12 units, well now you can easily strip the evasion off of things. And, as mentioned by speedbump, like in HBS btech multiple target suppression fire, aka shooting 1 shot at every visible unit with an early activation to strip 1 evasion (or add 2 activity level, which eventually is the same thing), is a very valid tactic. In late game HBS, the standard tactic becomes to take lots of damage by getting fortified in woods, as evasion becomes less and less reliable versus large numbers of enemy units by design of stripping evasion, but fortified cover doesnt lose effectiveness unless you get melee attacked/knocked down.
I would like to point out that you still get a chance to check for a reaction, no matter how many units attack you. It's still randomly determined, however after a certain point, the chances become slim. But, if you're rolling well enough, it can still happen.
But, yeah. This would mostly be for small unit actions. But, that's the level my group tends to play at, to begin with.
If I'm gonna do a proper Super Robot Wars mass unit engagement, I'd probably come up with something much simpler, like maybe a simple line or box to track the number of times a particular unit has been engaged against/declared against, and you roll against that. But, Super Robot Wars has its turns taking place as the course of a full minute, which is SIX BT turns.