You don't. Instead just ensure apcupsd is up and running (and all configured)
before plugging in the serial cable.
Sure - what I am concerned about is how vulnerable the system becomes. Will everything come up in the right sequence when the power comes back? I am afraid the answer will be "usually"...
Oh, I get the point now.

Yes, things do work 100% in the correct sequence.
But only if the correct UPS cable is used!
Which means.. the reason it switched off when you plugged it in earlier can only be that the wrong cable was used. Bummer.
Cheers