The first part of the problem can be trivially addressed in a DSP based exciter by noting that the envelope of any modulation scheme is trivially bounded by sqrt (I^2 + Q^2) at baseband.
Here is a band limited square wave (The pathological case for SSB):http://www.exponent.myzen.co.uk/Bandlimited square wave.png
Here is the calculated envelope by this method http://www.exponent.myzen.co.uk/Baseband_I_Q_Mag.png
And here is the calculated RF signal plotted on the same plot as the above calculated envelope: http://www.exponent.myzen.co.uk/SSB_RF_Baseband_envelope.png
Thus a limiter operating in the DSP on |Z| where Z is the analytic pair will suffice to limit the eventual RF output at any desired level.
There does need to be a feedback path returning at least VSWR, or more usefully a sample of peak drain voltage and current to the modulation DSP so it can adjust the limiter threshold level to keep the PA in safe operation, but extending this to an external amp while not particularly tough is not something the market expects (A power sampler head cabled back to the exciter).
There is also a potential issue with tube amplifiers here in that they can quite happily match into loads quite a long way from 50 ohms, where the power sampler would probably assume a 50 ohm match.
The takeaway from this is that if ALC is employed, the external amplifier should have as much ALC bandwidth as possible, so as to have minimum effect on the loop dynamics implemented by the exciter, certainly the ALC output should have a bandwidth at least or the same order as the modulation bandwidth.
Better would be for the amplifiers to provide a sample of the internal directional coupler output at a standard level on a pair of BNC or similar, if everything provided samples at say 0dBm = full rated power (or maximum rated reflected power), then the rig could use these samples to actually close the loop in a sane way with more or less fully known loop dynamics.
73, Dan (Who is designing a homebrew radio using these methods, hence having the plots available from my development simulations).