Keith, that could be possible, though you may be the first person to think of it.

But has Mallie ever been an *established* diminutive of Malcolm?

"Mallie" is well documented as a woman's name in Scotland. Both Burns and Cunningham used it..

If the *speaker* is a woman addressing her sweetheart Malcolm, the song becomes even more bitter, like some of Sassoon's Great War verses indicting women as encouragers of war.

Imagine a young woman urging her beau to go to Flanders to die while she sits on the sidelines snacking and drinking!

A bit much, I think.

If the speaker is a man, and so is "Mallie," then you have one friend suggesting they both go to fight and die (and snack and drink).

Conceivable, of course, but such songs are traditionally spoken by a man to a woman.

(Not that you should be stopped from imagining whatever makes the most sense to you.)