Why can't timing be handled external to the scrambling or descrambling operations? Look at the time, do the crypto, then check the time and sleep as necessary so that this particular operation takes about as much time as the worse-case. Something like:

  cryptofunc x = do
    begintime <- gettime
    result <- purecryptofunc x
    endtime <- gettime
    sleep (worstcase - (endtime - begintime))
    return result