If I had the money and slight hint of light at the end of the tunnel, I'd love to do something like this:
http://forums.vwvortex.com/showthread.php?4601413/page1

Obviously it's cake compared to our situation since the R32 comes AWD, but it's the only thing I could find in conversions.

 

Take a look at the rear end from up under a 350z. I bet you could work out dimensions to bolt that puppy up under the Max. I understand though you're focused on the IRS and not RWD.

Yeah, 350z rear end....cut out the spare tire well...then there's the gas tank and.....yeah, it just might be doable. I wonder about the exhaust...perhpas route it out the side the car depending on drop of car.

Nothing so bold could I take on....but I'll have to show you pics of my next project....after I get my '89 turbo back on the road. I can get you measurements of the rear end from a 350z if you like...I've said too much!

 

To me, it always seemed like Nissan could have given the Maxima an IRS with just a little more effort and $$$. It looks like you could split the rear beam in two and mount them to the body where that horizontal link at the center of the beam (forgot what it's called) mounts to the body now.
But I'm sure not a suspension engineer, so...

 

Bend the beam.. haha


I want IRS like no other..