I'm happy to report that I was able to book two tickets LAX-LHR in Business (D) and BRU-LAX (W) for $2913 roundtrip -- basically half the W fare plus half the D fare. This was exactly what I had hoped for.

I was then able to use my 3 SWU for making 1MM to upgrade the W fares to business and (along with 30K miles) the D fares to first. We saved almost $700 per ticket over a Z fare roundtrip and get to fly one way in First -- a very nice use of the 3 SWU for the two of us (both 1P).