Some of the Keitley (2001 and 2002) meters use PV Ok and a high voltage MOSFET in the current source. Chances are one could use a similar circuit (maybe dep. MOSFETs ?) also for just the protection.
I don't see a big problem with the way HP uses the chain of PNPs for the protection. The main down side is a somewhat larger voltage loss at higher current. With low currents in the low µA mauch of the current can bypass the transistors and less voltage is lost. A relatively high voltage is only really needed for low currents / high DUT resistance. So the voltage loss is not that critical as it may look at first.
With higher test currents like 1 mA and up it helps if the actual current from the source is reduced when the protection engages. Otherwise it take sizable transisors (maybe even TO220 case) to get a sufficient SOA.
Ideally current sources are made different for different ranges. The DMMs usually have quite a large current rance (e.g. some 0.3 µA for large resistors to some 1 mA or 10 mA for low resistors). This makes it somewhat challenging to get that with the same basic circuit, just swiching a few resistors voltages. So ideally one would split the current source to at least 2.
The parts to adapt are the switches and the OP-amp used in the regulation. The HP3458 for example useds an expensive DiFet type, that is good for very small currents, but not really good for the higher currents due to drift and LF noise.
The 34401 use a AD706 as a kind of compromise - not super accurate (especiall with low voltage across the current setting resistor) and also not great with sub µA currents.
Today there is the additinal option to use zero drift amplifiers in this place. They at least keep the drift part low, though they are not ideal for high resistance.
I have shown a reasonable well working current source in another thread. For the current source somewhat better resistors may be a good idea, but the basic circuit looks OK, a bit more optimised for higher currents, but still OK also for low current. For the switching much of the bias and leakage current can be seen as part of the source current. What matters is only the drift of these currents, not the current itself.
The 34401 type circuit is not that bad either. It allows for more voltage, but is a bit more sensitive the switch leakage.
How good the BJT matching needs to be is hard to tell. In the first approximation I get an about linear effect (e.g. CMRR on the order of 120 dB, about linear for 10% of Beta*Va*Gm matching) from the early effect. So in that approximation only something like an 1 ppm effect on the gain, but not yet nonlinear.
Chances are the nonlinear part is not that relevant as only a small fraction of it. Another point is that the matching would be combined of the BJTs and FETs and resistors at the current mirror. Chances are the FET matching ( transconductance at a fixed current) would be the limiting part and not the BJTs. A similar, maybe worse effect could come from the PNP to provide the base current.
P.s. For the part of an linear effect from the early effect, I goofed up: I only calculated the linear part and thus only got a linear result
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. I think one would get another factor of 2/Va for the 2nd derivative, so something on the order of 0.01 ppm/V. Still the calculation is crude and the model for the Early effect used ( beta = beta_0 (1+V/va) ) is not that accurately describing the real world.