I was under the impression that the "no connectors/extensions" wasn't so much around power delivery, but around the high data rates (40 Gbps) and signal integrity.
A charger cable, that only does PD, could have a magnetic break in it without issues.
In my experience, MagSafe 2 did a fine job at staying attached when pulled directly from the port, and came unstuck easily (as intended by design) when pulled from an angle.
I don't remember ever having problems with the charger popping out when it wasn't supposed to. But I also always used it on a desk, rather than sitting it on my lap.
A charger cable, that only does PD, could have a magnetic break in it without issues.