I can't find a mod like this anywhere, but I assume it would be simple to make.
Too many times I'm trying to push my way down a narrow corridor, all the while getting jostled by all my companions. Then I thought, it would be really nice if I could just walk right through them instead.
Is this possible?

Nope, collision information is setup through the NPC skeletons. Short of using tcl, there is no way for the player to pass through something which has physics unless they are pushed through by some external force which would of course also push the companions. Like in reality, you can't simply walk through people, the clustering that occurs when you enter buildings is one of the things that are just part of having companions. Afterall the game wasn't designed with the player having as many as 5 or 6 others following them around.

This is one of the reasons I don't use companion mods.

the game mechanics weren't taylored around it and as vagrant posted, you'll get a cluster of them milling about the enterance, especially in small cave enterances.

Unless the map was revamped to make every cave and ruin / fort / city merge into the same cell (probably not possible in terms of how much computer power it would need to manage all of that) you will have no work-around for this problem, but just use "TCL" when things get a bit crowded.

What about a spell which turns yourself into a ghost, but somehow retains the look of your model?

Vagrant and xenxander both proved fixes for the proplem. TLC is a console command that stands for "Toggle Collision". Its like "noclip" in other games. Open the console, put that in (tlc) and then move through your companions. Once you are on the other side, turn it back on. You don't need a ghost spell, just use the console.

Sorry I should have responded to that. I tried tcl but it only allowed clipping through objects, not people.