When battle regimen mode is active, support abilities/spells should not be locked out. This pertains to things like Taunt, Provoke, Second Wind, Cure, Chainspell/stoneskin, Accomplice, etc etc. Although with practice most parties should be able to execute BRs within a very small window, there are times when one or two members might be slow in getting their actions in the queue. In those situations we should still be able to perform support functions rather than stand around idle waiting for the BR to close.
Another way of looking at this is to restrict battle regimens to use only weaponskills, offensive magic spells, and the basic attacks while making support abilities non-BR compatible.