Carry we who die in battle

Solution 1:

There is no way to justify we there according to conventional rules if it is supposed to be the object of carry. It should be us:

Carry us who die in battle over land and sea.

The reason why this writer mistakenly used we is probably as follows. The relative pronoun who is the subject of the relative clause, and who refers back to we, so a hint of "subjectivity" might cling to we as well, in the subconscious of the writer. But this is not done in conventional English.

What Janus says below could be another reason: there are some well known phrases where both the antecedent and the relative pronoun are the subjects of their respective clauses (main clause and relative clause), in which case we who would be correct.

Solution 2:

Song lyrics are subjective and more dependent on sound, scansion and rhythm than sense, let alone grammar; that is why ELU prefers not to deal with them. This specific question, though, is a reasonable one. There is just one possibility. A Norse Valkyrie might sing the phrase to mean "We carry those who die in battle..."; if she can't get Wagner to write her theme tune, heavy metal would be a good second best.

(99.9% of English users would say "No, has to be us", and I've never properly encountered such a construction. But I recall a hymn starting "There is a book, who runs may read", and hearing a sermon about the grammar. Sadly, I wasn't listening closely enough to enligten you some thirty years later).