Safe "button"

How about a small squeaky dog toy she could squeeze?

I'd advise against using an electronic method as there's a lot that can go wrong with those.

A small air horn would probably be the most robust and reliable, the type you squeeze that used to be placed on bicycles.

The problem with squeaky toys is volume (and in some cases durability/reliability)

When submissive person have problem like that, its best to keep her hands free, not tie them at all and let her to stop, take gag ball out. Your duty as her Master is to keep her safe in any situation. If she still want some type of bondage followed with gag ball, then best way is to shake her head as a sign to stop everything.

Good master

You sound like a great Master. It is always important to keep your sub safe no matter what (speaking as mrdare the sub). My suggestion would be to find something like this:
https://www.amazon.ca/Allures-Illus...503?ie=UTF8&psc=1&refRID=QBBFYZP80F09AJZMCRGZ

The airhorn also would work but i feel like that may be too loud as airhorns normally are. And you dont necessarily want something the neighbors can hear. While her safety comes first i would try finding something like that button which could be held in one hand and pressed with that one hand. That way if she does still want her hands tied up she can have them so.

Have fun and stay safe

This type of situation always makes me nervous.
The concept of the safe word really is there to indicate when cries of pain or protests etc are "real" as opposed to being part of the play. The problem with any hand held device is the situation where it is dropped or dislodged so as to be unusable, or as already stated in the case of an electrical device it fails for some reason.
But its not just in your situation, any time a gag is used the potential for trouble is there. I can only agree with Master Vagrant that you need to be extra careful and possibly keep her hands free. I have heard of using facial gestures such as rapid eye blinks but really in the end you have to be extra vigilant and attentive to signs of real distress.

How about a rope that is close enough for her to grab and pull that rings a bell?

When my previous partner gagged me for scenes, the signal we worked out was that I would hold onto a bright red squishy stress ball; if I needed to stop the scene, letting go of the ball was my way of saying "RED!" when I couldn't otherwise speak. It worked in nearly every kind of position or restraint, except for laying on my back with my hands cuffed behind my back. It required him to make sure I was still holding onto the ball...but that's really a minimal level of awareness for the amount of trust I was putting in him.

Anyway, good luck, I hope you find something that works for you

I do like this stress ball idea, it has a kind of built in fail safe! Nice and simple, great tip!