I was wondering if it was possible to swap out the steel axle (which is really more of a support beam) for an alloy one.
Not really, but if you were to determine the depth, or, the length, of the rubber strips in each end of a Dexter full axle, you could, using Andrew's advice suggesting a cutoff wheel and cooling water, cut the ends off of a Dexter... or,
You could cut the Flexiride steel square tube suspension unit out of it's mounting (there's surprisingly little welded attachment) and insert it into an alloy square tube as you've pictured a few posts ago.
You should see a Flexiride unit up close to see what I'm speaking of.