Unless I'm mistaken, DTR is kept active at all times while the computer is using the modem. Its only function is that the modem is to hang up if DTR becomes inactive for 2 seconds.
So, if DTR becomes inactive for only 0.5 seconds and then becomes active again, what happens then? If the answer is nothing, then that could be interpreted as a signal to switch the modem to command mode.
Nobody can trick you into toggling DTR, so that should be fine, no?
3/end