Yeah, I don't see any reason why this weapon idea wouldn't work with your costume. As Nos said, with steampunk, just about anything goes. It's retro-futuristic. So weapons that are old or new can be made to fit. Just as long as they match the esthetic of the rest of your costume and props. Make it looks sharp and metallic. Maybe some battle-damage scuff marks. Maybe a touch of blood splatter. And of course, be sure to slap a cog in the middle or something to make it steampunky.
'm reminded of those weapons in Underworld that were like throwing stars (although this weapon is a bit smaller and different from your idea). They start as discs, and when the Vampire Death Dealers toss them at their Lycan foes, the blades extend.
The point is, these weapons are supposed to be modern, but they are designed with an ornate aesthetic because they were crafted by ancient vampires with a regal bloodline. Proof that any sort of weapon can be cool to use against supernatural foes, as long as the design fits the style of the monster hunter.
Oh, and the glaive from the Blade movies... duh. Just realized that's what you posted above. I believe that's the "Cyclone" glaive that Blade used in part 2 (okay okay, I looked that up). But Snipes sure made this boomerang blade look cool, didn't he?
zombombie, I think the challenge for you just might be making the foam boomerang look like metal. A bronze color would surely fit with your theme. But if it's not shiny enough, people might mistake it for brown wood.
Silver might read better as a blade, but I guess it depends on the sort of paint you use. You could paint some simulated highlights onto the surface (look at pictures of very shiny metal for reference). Or glue some metallic bits on there to reinforce the idea.
Another idea to show age in your weapon is to add some rust, which would look cool in a steampunk way.
Good luck!