How much power can a 20AWG USB cable safely handle?
2m (6-ish feet) of AWG20 should have a resistance of about 0.1 Ohm, assuming some inefficiency in cable make up and "wear and tear" over time. To and from would be 0.2 Ohm.

Which means that extension cable, in its wire, will create a voltage drop of up to:

V = I*R = 3.65A * 0.2 Ohm = 0.73V

That's probably acceptable on a 16.5V budget, as it's slightly less than the normal 5% minimum margin taken on external voltages. Most devices would/should work with a 10% margin on the external voltage.

The power dissipated in AWG20 would be:

P = I^2 * R = 3.65A*3.65A * 0.2 Ohm = 2.7W

Over a 2m cable that may just get a little warm, but should be fine, as long as the cable is made properly. That's where using USB extension cables for such manners of current may fall down a bit, because the main application 5V/2A would not account for 1/4th being lost in the cable. But, they'd have to be very old fashioned in their making of the cable for the slight warming to be seriously problematic.

One question does come up though: Why don't you just extend the power plug side with a proper coaxial power wire. Those come in current carrying capabilities up to 5A with limited voltage drop.