OK, some hot weather prompted me to get going on this, and I have my first results.
Conditions in my garage were 91F and 37% humidity for the test.
I collected sweat from both forearms, and measured the TDS (total dissolved solids) from each. This is measured in PPM NaCl:
Left: 2580
Right: 2302
I then mixed the collected sweat, and measured 2440 PPM. I will use 2440 for the calculations.
Using the formula (modified for units) from the Gatorade sports science web page to convert forearm sweat composition to whole body composition, I get the following:
Whole Body mg/L Na+ only = (0.57 * (forearm sweat NaCl PPM) + 618)*.393
This works out to 789 mg of Na/L whole body sweat. (959 mg/L from forearms.)
This seems to be right in the middle of the bell curve for saltiness, and I thought I was more salty than that 
The Precision Hydration online sweat test puts my electrolyte losses as ‘high’.
I will also do some sweat rate tests. I did one concurrently with the above tests, but since I was off the bike a lot it’s hard to say what the conditions for that test were. I sweat about 2.25 liters over 2 hours, 1 hour of which was riding in the hot garage, the rest was inside at about 76F.
I’m pretty happy that the left/right arms came out relatively close. I plan to do some repeat tests in different conditions and see if I get repeatable results.
Some things I have done to try to minimize sources of error:
- Wash arms after onset of sweating (Thanks @PrecisionHydration for the tip!) I rode for 15 minutes, then washed/rinsed my forearms. Final rinse in reverse osmosis(RO) water then dried them before wrapping them to collect the sweat.
- Added RO water to some plastic bags to check to see if either the bag, the plastic wrap, or the rinsed/dried paper towels would add measureable solids. The plastics didn’t add anything, and the paper towel only added about 6, so I’m ignoring that for now. (I guess I should have subtracted 6…)
- the paper towels I am using are soaked/rinsed in RO water, then dried. Without this step they add significant solids to the water.
- Checked the TDS meter with a calibration solution - it measured within 2.5% of the calibration solution. I did not adjust for this, but it measured the standard at 973 instead of 1000, so if I scaled the results for this, it would increase the salt content slightly.
- Checked how the TDS meter measures NaCl vs KCl. At the relative concentrations, the ‘TDS factor’, or the conversion from the measured conductivity of the water to the TDS value is within a few percent, so I am not trying to correct for this, but I do not think it is a significant source of error.