… In extremis you might pop the brakes if you’re wildly out of control, but even then you’d probably only be stopping the speed increasing rather than reducing it.
…
In my experience spoilers are rather less effective than airbrakes on glides, but this may just be the implementation in the gliders concerned.

Depends on the glider concerned. Any ‘classic’ wooden glider was built to the requirement of not exceeding Vne in a vertical dive with airbrakes extended, so will in most cases suffer a reduction of airspeed when airbrakes are deployed, rather than just stopping the increase (.e.g. 45 degree dive above Vm with airbrakes fully open will see a decrease generally). Same goes for some modern ships, but only in very shallow dives (a lot were built to the certification requirement of not exceeding Vne in a 45 degree dive with full airbrakes).

Spoilers are as you say quite ineffective on most gliders, and require sideslipping as well (T21, Rhönlerche, etc.) to properly reduce L/D. There’s hardly any of them used for that reason after the 1960s.

There is another form of airbrake, apart from the vertical one, hinged one at the back of the wing, and the spoiler system. Quite a fair few of aircraft from the 1940s to 1960s have peddles: spoilers hinging outwards from the wing (often above and below) on arms. Compare the T34 for instance.