No, it'd probably be weaker, heavier and a ****TON more expensive.
Billets have a grain (tires at 100+ PSI require a lot of strength, a billet may shear off along the braking surface), and extruded rims can be double wall (try machining that). Plus, unless you came up with some clever way to get a 29"/700, 26", 24", 20", 16" and 12" out of the same billet, there would be an insane amount of waste.
Aside: Back in the day Keith Bontrager used to take 40h tandem 700c rims, split them and then roll them down to make beefy 32h 26" MTB rims. Whatever fabrication skills you may have, you're never going to touch that.
Originally Posted by his wikipedia page
In 1984 he cut 700C (ISO 622) 40-hole Mavic MA-2 tandem rims to the circumference of a 26" rim, re-rolling them to create a 32-hole 26" rim.
E: Crap, this is the Scrod thread. Sorry man, hope you're already in SD and this blows over.