In aircraft where the thrust reversers are not locked out when airborne, is it permissible to go around after reverser deployment? In general, a go-around, in a jetliner, must either be performed before thrust reverser deployment, or not at all.

This is because attempting a go-around after reverser deployment may result in loss of control of the aircraft.
Loss of control is a possibility because one or more of the reversers could fail to stow during the attempted go-around, potentially resulting in an asymmetric-thrust situation beyond what most jetliners are designed to handle.
This is because the reversers take several seconds to fully stow once deployed, and the aircraft could easily lift off before this happens, in which case any not-yet-stowed reverser buckets or clamdoors will be blown back to their fully-deployed positions by the aerodynamic forces on said piece(s) of reverser hardware.
Lifting off is relevant because, when the aircraft lifts off, the reverser actuators are automatically locked out and depressurised.1
The reversers are locked out when airborne to minimise the risk of an uncommanded in-flight reverser deployment, which, for aircraft not certified for in-flight thrust reversal, is generally bad.

However, some aircraft are capable of, and certified for, the safe use of reverse thrust in flight (typically to steepen descents by using reverse thrust as an airbrake [DC-8, Trident, Concorde, C-5, C-17], or to shorten the landing roll by deploying the reversers while still airborne rather than having to wait until after touchdown [Trident 1C, Il-62, Tu-154]).

To allow this inflight use of reverse thrust, the reversers on these aircraft are not locked out when airborne.
As such, the reverser actuators remain pressurised even after the aircraft lifts off.
Consequently, even if the aircraft lifted off before all reversers had fully stowed, the reversers would simply continue to stow.
As a result, lifting off with one or more reversers still in transit would not produce a significant risk of a severe asymmetric-thrust situation.
Even if, for some reason, a reverser did fail to stow, resulting in thrust asymmetry, aircraft certified for inflight reversing are required to remain controllable even with full forward thrust on one side and full reverse thrust on the other, at least for long enough to shut down the reversed engine.
This would appear, for aircraft certified for inflight reverser use, to essentially eliminate the danger of loss of control (absent other serious failures) in the event of a go-around after thrust-reverser deployment.

This would be especially important for aircraft like the Tu-154, Il-62, and Trident 1C, which routinely deploy their reversers before touchdown; if going around after reverser deployment was prohibited for such aircraft, this would seem to completely preclude very-low-altitude go-arounds with these types.
Are aircraft certified for in-flight thrust reversal (and, thus, not equipped with an in-flight reverser lockout mechanism) allowed to go around even after the thrust reversers have been deployed?

1: There is generally an override switch in the cockpit that can be used to restore pressure to the reverser actuators even when airborne (its main use is to force the reversers to remain stowed if one or more should come unlocked in flight, or to restow then in the event of an uncommanded in-flight deployment), but locating and actuating this switch would take precious seconds which might not be available in the event of an asymmetric reverser deployment at low altitude in an aircraft not certified for the inflight use of reverse thrust (especially as, during the time spent with their actuators depressurised, the deployed reverser(s) would be blowing back to the fully-deployed position, taking them further and further from the stowed position).
An aircraft approaching to land with a reverser deployed or unlocked, particularly one that's deployed unintentionally, will be at idle power, or shut down on the deployed reverser.  

A go-around or missed approach is conducted in the same manner as if the engine is shut down for any other reason.