Yes, the effect can be estimated quite easily. Also, keep in mind
that the 0.13 nV is the RMS noise, so the peak to peak excursions can
be around six times that, or almost 1 nV p-p. If the FS is 1 mA, then
it's about 1 ppm - one count on a six digit DVM, or ten times more
with each additional digit. In reality, one ohm would be commensurate
with a higher range like hundreds of mA, so the noise voltage would
be much further down as a percentage of FS.
Also, it's hard to amplify up from a very small FS value up to the
native range of high resolution DVMs without adding even more noise
and thermal effects, so it makes sense to use more resistance - and
undesirable burden - to get a decent FS signal that needs less
amplification, but the resistor noise would be higher. It's part of
the tradeoffs needed in design.
When comparing current range performance to that of the voltage
ranges, you should compare to the voltage range that is closest to
the FS burden in order to take into account the noise and error
contribution of the required amplification. Also, the specs should
include the effect of amplifier bias current, which introduces
additional error on high sensitivity (low I-FS) ranges.
In the pursuit of ever-higher DC resolution, once you get down to
where individual to tens of nV are significant to your signal,
realistic resistance values contribute significant noise. If your
signals are big, then it's insignificant. That's one of the reasons
why the native range of the DVM is made as big as possible.
Ed
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