I've been thinking - a lot of fans will maintain spinning at at 5v but not start at that voltage.
I'm no electronics expert, but maybe you could start up a fan at 12v, and then when it's electrical resistance drops and the current flows more that'd close a relay, switching to 5v or less? It'd be at 12v for maybe a second when the PC is turned on.
Has anyboby tried this? Is there anyone out there who thinks it could be done? I'd be very interested to hear following up this idea since I have a couple of 120mm fans that I usually end up push-starting