First off, make sure the squeek is coming from the hub and not disc rotor rub. I've been running a lefty for 3.5 years and have never greased the lock-bolt washer.
How old is your hub? The bearings might need some grease, which can be easily accomplished on the inner/larger bearing by pulling the wheel off, taking off the rotor, gently prying off the black ringed bearing grease seal and pushing some fresh grease into the bearings.
If, for some reason, the squeek is from within the hub, be aware that the outer cap has reverse threads=righty loosey- lefty tighty. Also, there is a very low torque for the outer cap of 3.4 Nm. In other words, basic hand-tightness. Once you've removed this, go ahead and check the grease situation of the outer/smaller bearing.
I've found that some sealed bearing cartridge manufacturers have been very sparse on the quantity of factory greasing. You want to pack the things with high quality bicycle grease, preferably with teflon.
Also be sure to clean off and lightly re-lube the lefty fork spindle before re-installing the wheel. The problem may be originating here.