Found: Token Valve Extender w/ Campagnolo EPS Magnet Plug Port
Token felt your pain and created a simple valve extender with an open end sized to hold the magnetic key. Just slide it in, and you’re off without fear of losing it from a jersey pocket. It’s CNC machined from alloy with a tiny steel bolt inside to hold the magnet. Weight is just 1.6g excluding the EPS magnet button.
Retail is just £4.99, EPS part not included. Thanks to Tom from i-Ride for sending this in.