I've spent a few years working the pharma angle.. Making water to USP standards (DI, or 18 megohm water)
Salts tend to be the big effector for resistance, in my experience.. But... I have no info one way or the other regarding sugars, so I too would be curious.
I would think that in order for a resistive measurement system to work tho, the user would have to measure the resistivity of the source water (after any salts / buffers / gypsum / water additions have been added, and thoroughly dissolved.) to get a baseline reading, and to calibrate for the batch..
But, I could be wrong..
You could build a rig that holds the probes a set distance apart (say 1 cm) then test resistivity of the source water. Then test a wort pre-biol, and post-boil to see if you get different results..
If you do, then perhaps a controlled experiment making a (say 1lb DME in 1 gal water) vs smaller, or larger quantities in the same water source to see if the probe scales readings in a meaningful way.
Sorry if i'm rambling.. Too many homebrews