Well, I went down the rabbit hole a bit on trying to understand the PCV and vacuum setup on the RX8 so I would know where to put the catch can etc. Well... turns out it seems like the 2011 S2 diagram and my 2009 S2 car are a bit different.
The FSM diagram only shows two connections to the oil fill neck; the red and green lines shown below:

The green line comes from the pre-throttle accordian to provide fresh air at atmospheric pressure to the sump, which is drawn out through the red line at the top of the fill neck and is shown as going back into the manifold just after the throttle body. The blue line that I drew on there does not exist on the diagram, I just put that on there to show approx where a third mystery vacuum line connects.
Here is the engine itself (with the alternator, upper intake manifold, throttle and accordian etc removed):

The green hose is exactly as shown, running from the accordian to the bottom of the oil tank right above where it enters the center iron. This tube definitely had oil getting pushed back through it and leaking into the accordian, filter box etc, so this is the tube that I put the oil catch can in the middle of (pictures to follow).
The red hose is confusing - it comes off the top of the oil fill neck, but instead of running to the manifold just after the throttle body, it runs down the passenger's side of the engine to a "T" that is then connected to nipples going into the two primary runners. Sure, this is still a vacuum source, but not only is this a much different location than the diagram shows, but I also think that this location on the primary runners are the jet air nozzles to help with fuel atomization. However, this connection in the diagram appears to go to the second connection in the accordian (also roughly atmospheric air, but not from the sump). So... functionally this should work similar to the diagram, but why are they different? This also raises the question... where does that second line from the accordian actually go to if it's not running to the jet air connection?

Finally... what is that blue line, and where does it go? Well, lets see... it heads under the oil tank, so off that comes too, to find a mess of hoses and vacuum lines.

Well, it connects to another vacuum port on one of the OMPs. Not sure why it wasn't shown on the other diagram, even if just as an arrow pointing to a call to another diagram.
Well, since oil was definitely coming through the green line into the intake accordian, I installed the oil catch can in that line. Cue the bracket-making (nowhere near as nice as the "Bad Obsession Motorsports" guts).


And here it is, tucked between the fuse box and ABS pump. The original green line to the oil tank was long enough and shaped close enough to just re-use going to the inlet port of the catch can, and 3/8" piece of left-over hose was used to run from the outlet over the top of the accordian back to the original connection.

Now, we'll see how the day at Waterford on Sunday goes, and whether any more accumulates here etc. Hopefully its not excessive blowby, but if it is... well... I may just need to be catching and draining oil for a while until I'm ready for a rebuild.