This is going to happen to you if you get mcd/sheeped etc, the thing to do is use targettarget targeting and have a mechanism for switching who your target leader is.
You would have change leader macros for each character to be the target leader of the group, so for a 5 box group each toon would have 3 leader macros assigning target leader to different members in addition to the normal follow macro, I recommend having the leader macros in the same order as your normal leaders group sequence so you can remember who is who quickly as in all likelkihood you'll only switch once and then back to your normal leader (hopefully):

change leader macros:
your brain - you have them on your brain as well as it's handy for other things
macro1:target=toon1 keyed to F1
macro2:target=toon2 keyed to F2
macro3:target=toon3 keyed to F3
macro4:target=toon4 keyed to F4

toon1
macro1:target=toon2 keyed to F2
macro2:target=toon3 keyed to F3
macro3:target=toon4 keyed to F4

toon2
macro1:target=toon1 keyed to F1
macro2:target=toon3 keyed to F3
macro3:target=toon4 keyed to F4

toon3
macro1:target=toon1 keyed to F1
macro2:target=toon2 keyed to F2
macro3:target=toon4 keyed to F4

toon4
macro1:target=toon1 keyed to F1
macro2:target=toon2 keyed to F2
macro3:target=toon3 keyed to F3

Why only 3 macros? Well the brain is MC'd/Sheeped/Dead so can't do anything anyway and you don't include the new target leader which is the new brain. Plus you already have a follow macro targeting the old brain

So your main gets sheeped or mcd and you hit F2, Toon2 is now the target and you control from the toon2 screen, since all your macros use targettarget for DPS you can continue the fight instantly. If you have some sort of PIP swapping like in keyclone you can also swap screens to the new main by linking the F1,F2,F3 and F4 keys to the screen you want to swap. After the fight just hit your normal follow key assigning target to your brain and everything is reset.

Question: Simulacra, you are a moron why do you think this is better than a failover follow macro?
Well I'm glad you asked that question because I love to opine.
Answer:do you want to leave the choice of brain to a macro which doesn't leave you a choice of who the leader will be given that optimum party leadership is a situational judgement? It processes like this: if party1 is outofaction, failover to party2 if party2 isoutofaction failover to party 3 etc etc. Looks good! the problem is in the heat of battle you may not want toon2 to be leader because he's stuck with the whelps getting beat on and you much rather have toon4 who was feared into a corner and no mobs are on him.