Contemporary songs from the time of the event are primary sources. And in some cases some cases modern songs can even effectively incorporate references to or quotes from earlier songs which were sung at or around the time of the event - as with "The Band Played Waltzing Matilda", of course. It's also been done with "Tipperary", and others.

My feeling is that a modern song written about a historical event is generally better not to make out that it's an old song written then, but perhaps have a sense of looking back on it. (As is the case with "The Band Played Waltzing Matilda", of course.) Though a great song like Anderson's Coast doesn't do that - however it also doesn't try to copy the style of the time too closely. It doesn't feel "period", while at the same time it's true to the facts.

I wouldn't think that modern songs about a historical event would help specifically in writing another song (apart from the way you wouldn't want to write a too similar song). Songs about historcal events in general might be useful, of course, as giving hints as to how to do it, and how not to do it.