Hey guys, just wanted to follow up on this to share what I've learned, in case there are others in my same position.
I've just come back from the US Embassy where I met with a couple members of the IRS department there. I took with me some notes from this conversation, specifically the references to IRC section 402(b) and 409A, and questions about the UK/US tax treaty Article 18, and form 8833.
The basic gist of my meeting was: "stop worrying so much."
The IRS employees I spoke with (btw to my surprise they're a friendly, down-to-earth bunch!) told me that since I am taking part in a standard UK pension, which behaves like all regular UK pensions do, I do not need to worry about reporting my contributions, my employer's contributions, or any growth in the fund on my federal income tax return. It is only when I start taking distributions from the pension that I will start to pay tax on it, and at that point it will depend on which country I am resident.
I asked them what happens when I leave my employer and roll over the pension into a regular UK personal pension. So long as it's a direct rollover into a pension that behaves the same way, again I don't need to worry. They said that the idea of treating a personal pension as a foreign trust, and having to worry about foreign trust reporting, was a highly conservative stance to take ("paranoid" in fact was the word they used).
Another notable quote from them: "I have never known anyone to file a form 8833 for a UK pension." Form 8833 comes in to play if you are taking an abnormal position with regard to the tax treaty, and omitting a UK pension from your income is not an abnormal position.
Again, I was told that the only place I need to report my UK pension is on my FBAR, and form 8938, if applicable.
Guya, thanks very much for your advice and help with this, it was very helpful to have some notes to take with me from someone who knows what they're talking about. I appreciate that as a tax professional it is in your best interest to take as careful and conservative a position as possible. But it definitely put my mind at ease to learn that the IRS is not nearly as draconian and punitive as one might think. They want you to be able to invest for retirement and the tax treaty is there to help you with that, and avoid double taxation wherever possible.
If you are an ordinary person with an ordinary income taking part in an ordinary UK pension, there's no reason to freak out or think that your taxes have just become incredibly complicated. File your FBAR, file your form 8938, take the foreign tax credit, and Bob's your uncle.