I don’t think what you’re asking can be done. If the EX2 shuts down when the UPS hits 10%, it’s no longer communicating with the UPS via USB as its processor isn’t running.
I think you may be inventing a problem where there is none. If there’s an extended power outage and the UPS runs completely flat and shuts down, the EX2 will start back up once power is restored, assuming that your UPS will automatically restart in that situation.
The only time you will have an issue is if power gets restored after the UPS batteries hit 10% but before they go so flat that the UPS shuts off completely. This doesn’t seem very likely and if you think about it, it’s no different to how a PC or server connected to the UPS would behave in the same situation.
Your other option is to forget the automatic shutdown and just let the EX2 hard power down once the UPS batteries run flat. Whether or not you want to risk that is up to you.