Easy, something like twilio to grab all SMS messages, and react on either any message being sent, or a specific message being sent. I guarantee you someone in jail can find a way for someone to send a text.
Looks like you didn't read OP carefully.
Any idea that does not implicate a "trusted 3rd party", now ?
Just you, your fat wallet, and some people determined to gain access to said wallet.
A trusted 3rd party? Wut? Firstly, how is anyone gonna know you used twilio, and secondly, why do you have to borrow someones phone? The gang heads can get cell phones in jail, why can't someone with millions of dollars in Bitcoin?