Every party member except the Monk is a spell caster, but all of them except the Bard went multi-class. The best we could do is a Summon Monster 3, which gets us a Small Earth Elemental, max. And if you want it to do anything more complex than fight, you need to be able to speak Terran. That limits it to the Bard, who happens to be an NPC at the moment, since he's my character.
They do plan to bring him next time, which means smaller EXP for the group (adding the NPC means it gets divided a bit thinner), so it's not a popular move.
Now, the Mage Armor spell can affect the Ethereal Wall of Stone, and the Elemental can glide through earth like a fish going through water, but will the Mage Armor spell glide through as well? Tough question.
And if it does, won't it also glide through the Ethereal stone the same way?
I hate to be the kind of hard-nose Dm that shuts down creative ideas, but I'd have a hard time rationalizing that it could be that conveniently, selectively immaterial, if you see what I mean.
A perfectly valid solution would be to just walk away, acknowledging that there are problems in the world that they aren't yet ready to tackle. The effect of the blockage is that sometimes, when people in the area die, their spirits are trapped in and around the mortal realm. This means more Ghosts than usual in the area, and sometimes when someone should be dead their spirit can get back into the body and live again, at least temporarily.
Note that I'm not the one who came up with these limitations. The "Land of the Dead is Temporarily Closed" theme came from another DM in our game and he came up with the rules. All I'm doing is abusing it for the sake of a Halloween scene.
In any case, a Small Earth Elemental has a Strength of 17, so a +3 modifier. Needs to roll a 17 or better on the dice while he's there. Figure it can Earth Glide as fast as it normally moves, 20 feet per round, and figure that the spell area was 50 feet across (5th level Druid), so we'll call it a round lost to movement. That leaves 10 rounds of Strength checks to roll that 17.
Multiple castings will probably be called for. Bull's Strength would improve their odds considerably.
So yeah, if it can work at all, it has good odds of success.