Ok, I finally got a chance to put a borescope down the hole and take some video for you guys. The video quality's not great (had to improvise with my iPhone b/c the scope had no record capability). But it did the job. You can clearly see the plastic piece that's been added. It's substantial and it is epoxied in place. There is virtually no room for liquid epoxy to be injected into the driver's side cavity unless the viscosity were very low, which I doubt to be the case. There is no such plastic piece on the passenger side, meaning it would be easy to do the epoxy reinforcement on that side.
Here's a link to the video...
https://dl.dropboxusercontent.com/u/...0Video%202.mov
Having studied this, I am now 95% sure that the purpose of the piece is to reinforce the subframe and prevent cracking. Keep in mind that the piece was added into production 5 years before BMW released the epoxy repair protocol (in 2009). So it can't be a dam to make the epoxy method easier (contra Reddish) b/c no one was injecting epoxy into these cars back in '04. Instead, I believe BMW added this piece to stiffen the floor (a metal/plastic/metal composite sandwich) because they were hearing about more failures in earlier cars. Years later, as the failure rates of older cars increased, BMW began to look for a way to mimic the effect of the plastic piece in existing cars (obviously you can't add the plastic piece in to a chassis that's already welded together). The epoxy was the best available option. With the expanding foam barriers in place, you are effectively creating a plastic-like block between the sheetmetal layers, creating a metal/epoxy/metal composite sandwich. This would explain why they did not allow their techs to do the epoxy method on cars produced after 10/2004 - the plastic piece has the same benefit as the epoxy. If the subframe still failed, there was nothing left to do but replace the whole thing. This would also explain the much lower rate of failure of post-10/2004 cars. I only found 1 instance online of a post 10/2004 failure that I could verify, and even then there were no pictures or details to explain where it failed.
Summary: for e46's produced after 10/2004 (or thereabouts) that include the plastic block reinforcement in the chassis floor, there appears to be no way to fill the driver's side cavity with epoxy and no need to do so. That location has already been strengthened. However, it's still worthwhile to reinforce the passenger side mount with epoxy injection on a car that will see hard driving, especially one on a stiffer aftermarket suspension. That's what I plan to do at some point in the near future.
