Anyone care to gues sthe significance of an obscure Momma Cass song playing REALLY loudly in a Scotsman's bunker??

It applies to Locke/Kate....not necessarily following the actions of the rest of the group, but going out and setting their own path. When they want something done, they go do it. same can be said for Michael, in a way.

Individualism....

part of the lyrics say:

You're gonna be nowhere
The lonliest kind of lonely
It may be rough going
Just do your things
The hardest thing to do.

Kinda beating a dead horse here, but I think the lyrics apply to Desmond entirely. It needs no explanation. Read them or listen to it.

If he's been down there for years I'm sure he memorized every song and really enjoyed this one after a while because it spoke directly to him and his situation.