Shouldn't be a problem. I've been playing with MSH and whoopies for the last month or so (thanks, Duffy, if you're reading this!), since it allows for a shorter strap holding the hammock/insulation to the pack--less looping before tucking when I pack up in the AM.
First, I made a pair of toggles out of carbon fiber arrow shafts I had left over from the pack construction. Those did just fine. Then, I had the brainstorm of, "Wait! I'm already using mini 'biners to hold the Camelbak to the pack...why not try those!" And yeah...they worked just fine.
So, no, I don't think there's a minimum diameter (though I could be wrong), as long as your toggle will hold up okay against the compression forces on it. Solid bar stock should do just fine (if be a little heavy for the length--but we're talking "heavy" in a relative sense, not an absolute one).
Let us know how it works out!