I'm not so concerned about the fully discrete approach; like I said, if I had to, I could roll my own everything, and make an op-amp with built in voltage controlled limits. It would just take 20 transistors, at least as many resistors, and still need offset trimming.
As for discrete limiter blocks not including the amp, I already have a pretty good one of those (which is why I started this thread -- to do it with the op-amp included, this time). See attached. This one uses six transistors (one optional), offers a +/-20mV (or so) offset trim (referred to one input only, it seems), can be expanded to nearly unlimited inputs, has a buffered emitter follower output, and (as shown) also has a indicator output to show which input is active. For example, you could put this circuit into a lab power supply and use the indicator output for the current/voltage regulation indicator.
The transistor models are as shown, except for the starred parts, which have different IS parameters, to demonstrate the offset trim and matching over variations. I don't have those data offhand (the temp/sweep plots), but I think I had ran them.
Again, this type of limiter is best suited to use with an op-amp, not necessarily with high GBW, but high slew rate at least, so that the recovery can be relatively short; and especially when used with external compensation, because the feedback can be drawn after the limiter with no need for a second op-amp as a follower, and the slew rate will be a much smaller fraction of the overall time constant. It's worth noting, B-E avalanche will screw things up if the op-amp output goes more than +/-7V from the limit voltage, which is dubious for +/-15V applications, but *might* be acceptable on a +12/0V system.
So to reiterate, my "holy grail" here would be a circuit that does this (the voltage controlled, low offset, precision limit), with the buffered output (equivalent to an op-amp output), indicator output (optional but desirable), and with high (open loop, DC) gain and uncommitted differential inputs (equivalent to an op-amp input) for the signal input, without incurring more than transistors and jellybean op-amps in construction, and minimizing or eliminating slewing or windup type recovery effects.
Tim