I don't think Onslaught mode and dedicated servers would get along, kinda in the same vein of dedicated L4D servers not being insanely popular.
And I don't think EA wants to pony up the server support for PC. I played it on 360 and their servers are absolutely terrible, at least in the first handful of days after it hit consoles, so I don't think they even really want to put in the support they have right now for consoles.
For what it's worth, though, DICE did charge consoles for it so you'd probably expect a pricetag if it came to PC, too.