Something like this:
Coupled with those simple, 3.0-to-2.0 header converters, perhaps?
I also saw some no-logic (i.e. just splitting wires) 3.0 splitters, but the reviews on their pages aren't very comforting.
I'm assuming here that the "USB 2.0 devices" you're referring to are internal. If they are external, then you're probably going the wrong way here. Just grab an external USB hub and plug it to your external ports. Or at most extreme a PCIe expansion card. No need to mess around with internal USB wiring at all.