After re-watching The Empire Strikes Back after many years, I noticed that as Luke flies away from Hoth, he says to someone (probably Wedge Antilles)
I'll meet you at the rendezvous point
Or something like that (I might be confusing the words from when he tells Lando he'll meet him on Tatooine, but he definitely said something like that). Even in Princess Leia's briefing to the fighter pilots escorting the transports:
When you've gotten past the energy shield, proceed directly to the rendezvous point.
It doesn't sound like there's any room for interpretation there. The fighters are to head straight for the rendezvous point as soon as they're clear of the Imperial blockade. Even R2-D2 is under the impression that Luke's orders are to proceed to the Alliance's regroup point, and is surprised when Luke tells him they're headed to the Dagobah system.
Was Luke technically AWOL? Is there any canon evidence that can clear him of the charge of desertion (with Alliance equipment, no less)? Did he get clearance from anyone at all to go to Dagobah?