Looking at a telescoping antenna today it occurred to me that there was a way to stabilize telescoping mechanisms for the purpose of, for example, folding bicycles, with a bayonet-connector-like approach.
Specifically, given two coaxial tubes, one inside the other, you have two or more pegs in one of them (say, the inner) which engage slots in the other (say, the outer) when they’re close to the fully extended position; by rotating the tubes relative to one another, the pegs move into channels that move the tubes into a really fully extended position, working against a spring, and when the rotation is continued, the channel allows the pegs to slip into a detent, allowing the spring to relax slightly. The only difference from a BNC connector is that the action is reversed: the pegs engage when the tubes are nearly apart instead of nearly together, resisting compression on the column formed by the two joined tubes instead of tension. (If you need to resist tension you can do that better with a cable, which can be tensioned to hold the strut in compression.)
Optionally you can include a second analogous set of channels to lock the tubes in the fully collapsed position as well, or any number of other positions.
In this way a telescoping tube can provide a strong structural member that can be easily collapsed for storage or transport.