it's clearly implied that one should NOT use such "home-printed"
plates in the cockpit.

Given that there's no FAR requirement to use instrument plates at all
(current or not) for Part 91 ops, I see no legal ramifications at
all. Also, NACO itself publishes them on the web, which is implicit
endorsement for the use of home-printed charts.

The "not for navigation" appears to be CYA.