I think I have an idea that would both benefit players and make burying loot a mechanic worth interacting with.
First, the problem: I don't think your average player buries treasure. My family goes to bed around 7:30 pm every night, giving me 3.5 hours to play; and I do, virtually every night. But I don't feel that's "average." I have many games that I play with friends and getting in one to two hours every few nights is about it. So, in my age demographic anyway (millennials, "old millennials," Xennial, what have you), the idea that you'll bury treasure when being pursued and come back to pick it up later is absurd. There is no later. You spend 60 minutes getting the loot and grade five emissary, then 60 minutes getting chased. Maybe you drop off a chest or two sailing past an outpost, maybe you get sunk, maybe you don't, but either way we basically run out of time to retrieve and sell the loot.
So, why should I bury it? Setting aside that burying treasure didn't happen in reality, why would fantasy pirates bury treasure? Because it's their "bank." You bury it later in hopes of amassing enough to come back later, dig it up, and retire in luxury. The idea of lost buried treasure was that pirate X buried a bunch of treasure and died before they dug it up. So why don't map bundles persist on my captained ship? This would be great for players with less time to play. Get some treasure, bury it, dig it up in a later session to pick up where you left off.
The way I envision this being used is do a voyage or two, bury the treasure, come back another night to do another voyage or two, bury the treasure, come back another night to raise emissary, dig it all up, and sell at grade five. It doesn't really change the risk/reward balance; there is still loot on the ship to be stolen but instead of dropping it off at an outpost it's dropped off at a random island. You can still have the map bundle stolen, or lost when your ship sinks. (If the map bundle disappears I image the loot does as well, if you log off.)
May not make the map board any better, but it makes burying a mechanic some players would interact with.