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alan.bain:
You could try the diag:peek? -10,<option>,0 commands where option:
// 0 N/A
// 1  sdev,pp
// 2 temp
// 3 scale
// 4 custom aperture
// 5 Save state
// 6 N/A

If this works then it's likely the POKES 28-33 do as well.  If you *really* want to check look for the byte sequence A704 (magic 1191) in the firmware  e.g.
LANG=LC_ALL grep -oabP "\xa7\x04" firmware.bin and look for a hit in the bottom 64K; this is the POKE check.  In the 07-05-02 firmware it is at 46E2. Disassemble from 1 byte before this and you'll seek the POKE table dispatch routine which uses a table + 2*( zeroth arg + 4) like this

46E1       CMP     E2,#04A7; int(I2)==1191?
46E5       JNE     46EA
46E7       LDB     E1,#01 ; second arg is 1191 -magic on value?

;; Arg too big?
46EC       LD      1C,1E[18] // p=*(SP+1e) (orig p)
46EE       LD      1C,2[1C] //1c = *(p+2)  => command
// Table lookup
46F2       SUB     1C,#0FFFC ; +4
46F6       JLT     46FE ;; if <0 => -1
46F8       CMP     1C,#0025 ;
46FC       JLE     4701; <=0x25 (max tbl, allowing for shift of 4)
46FE       LDBSE   1C,#FF // -1
4701       ADD     1C,1C // table index
4704       LD      1C,470E[1C] ; base + 2* (zeroth arg+4)
4709       BR      [1C]
Hydron:
You could also run strings on the FW binary - if it doesn't spit out a bunch of stuff like "SCALE" "TEMP" "RECALL" (I think the menu stuff is mostly grouped together) etc then you're unlikely to have the options in the FW.
iMo:
Any hint how many digits the "scale" coeffs (gain, offset) allow? I can see in the 34420A manual the options, but without any indication on the precision of those coefficients..
Hydron:
7 digits with scale. Also you can use the custom aperture to get faster 6-digit readings than otherwise possible.

Edit: to clarify, you get 7 digits (no overrange though, so 9,999,999 count max) on both the output and the coefficients
iMo:
I've been using PLC100 with 6digits ("slow 6digits"), getting 1x.xxx.xxx digits at 10V over serial, and doing the "scale" in double in an MCU (sending the scaled and TCed data over BT). I have to double check whether those 7digits of gain and offset give comparable results with my process, a great option when not using the MCU, however..

PS: yep, "7digits scale coeffs" are not enough to scale the above resolution, 8 would be the minimum (see an example below)..
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