yeh could use a modifier
/cast [nomod] something
/cast [mod:shift, target=focus] something

then send all keys to the background modified by shift

/tar [mod:shift] focus
/assist [mod:shift]
/cast [mod:shift] something
/cast [nomodifier] something

or

/cast [nomodifier] something
/stopmacro [nomodifier]
/tar focus
/assist
/cast something

syntax may be off :P