Rod nuts stay tight because the bolt stretches a little and creates enough internal tension to keep the nut from backing off. I can't imagine a loose rod nut being kept from backing the rest of the way off the bolt by a palnut. A 3/8-24 rod bolt usually gets 35 ft-lbs. of torque, and a palnut might handle a maximum of 10 ft-lbs. on a really good day.

I've seen ARP rod bolts that needed 70-80 ft-lbs to get the specified amount of stretch on the bolt, which is how we measured the "torque" on racing engine connecting rod bolts.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!