the fin stalls and the rudder locks out
I have no experience, and therefore no comment about the KZ 7. Over the years, and with different flight test programs, I have found that a very few airplanes can be flown to this extreme condition. Certain Cessna 180/185's can get there, particularly as floatplanes. I've done it in a King Air B200, for which it is a know characteristic, and a certification special condition. In the case of these types, recovery can be accomplished with appropriate use of the rudder - it just was not a positively state as other types. But, the rudder did not "lock", or it would not be certified. In both cases, this was powered flight, idle power flight was more benign. The turbine DC-3 will exhibit unfavourable rudder forces with very large rudder deflections, which is alarming. The DC-3 was certified before today's more modern standards - I consider it an exception, for which competent training is pretty important. I have slipped 172's with flaps extended, and experienced the burble in pitch which is warned as an "avoid". Again, manageable within normal pilot skills, as long as you just keep flying it. I have never made any version of a 150 loose positive directional stability, and I have the most experience on them.
During testing of a modified Cessna Grand Caravan last winter, the authority required that I demonstrate a stall to the break from a 75% power climbing 30 degree bank turn in each direction, with full rudder applied in each case. (Yes, this is about a spin entry). The airplane handled it just fine, no rudder lock, benign recovery attitudes.
I find a sideslip to be a very useful tool for descent control, and have done it in many types with no difficulty nor alarm, though no, I would avoid it in a DC-3!