In my opinion I think that we should be able to spawn withers in the overworld since this pack requires quite a bit of power generation and a good way to generate power is with the nether star generator. For this to be feasable it would be nice to be able to not have to spawn the wither in the nether and instead be able to spawn it at our base in a wither-killing machine.
Does infinity have the color coded private enderchests? you could setup a whiter killing machine, chunkload it and make the nether stars go to your base.
As stated by, and for the reasons given, Booker is right. The Wither is not and will not be allowed to be spawned in the Overworld. Pseudonycat has given a great suggestion, and is in fact the method many players already use.
Hypothetically, if someone managed to spawn a wither in the overworld, would it be against the rules?
Do not attempt to spawn a Wither outside of the Nether. The precautions you take are completely beside the point, that is is not allowed. IF you were somehow able to do it, you would at the very least be guilty of violating the Server rules. Just Don't Do It. Really is the safest thing for everyone.
@wyndman I believe the specter dimension would work for that, along with a spacial storage cell maybe?
Not sure about the Dim-ID for the Spatial Storage, it could work. The Spectral Dimension is a great idea though. There is also the route I use with my Capacitor Seed farm. I set it up, in the Nether, on the 1st or 2nd and then take it down between the 28th and 30th each month. Little bit of work, but it's worth it.