This could be a legitimate idea save for the thought that rule breakers that become aware of this button, or random players that find this button, may hit with the intent to slow or otherwise modify the round in such a way that may make rebelling (or freekilling) easier.
Edit: Above statement is for an argument "If the button is a physical one like the Gravity button in ba_spacejail"
It could be a warden menu option instead so that wardens that are either experienced enough, or prefer the use of nocrits/minicrits for RED/BLU respectively, can use this without any interference.
The only major problem I could see is actually implementing it without something breaking, which is a common issue with Jailbreak modifications (experienced in Panda JB and various other non-Blackwonder servers).