I think this is probably out-of-scope for this thread, but I don't know if one exists for matrix. If it helps, when I use matrix I generally don't enter any advanced routing codes, but it can help to use "multi-city" to force a specific routing and to select specific flights. I always select "time bars" for the results page so I can select individual flights, and if you hover over a flight you can see the fare class. I think you are getting too hung up on this specific R class fare... widen your scope to see what the best available is... there was not a huge difference between R and I on the flights I found (about $150, which on a $3.5K fare is loose change). I find expert flyer an incredibly useful tool and even though I have been grounded for over a year now I still maintain my subscription. It is even more powerful when combined with other tools like matrix... as your specific case illustrates.

P.S. search +/- one day and you will find BA flights for under $3K.