FCOG,
You may be batting on a sticky wicket there. That's an Anglophile term for being in a difficult situation. The actual rear pegs are easy to replace with substitutes from eBay. I bought a pair of nicely turned alloy pegs myself recently, which were about $40 AUD shipped to Australia. The rear R peg mount looks like it may be an elusive beast to track down, although occaisionally XR parts turn up in the classifieds here. But if you want to keep the D& D pipe as well as pack your gf on the back, about all I can suggest is going to a local machine shop or welder and having them create a peg mount for you. That might turn out ugly though functional, or it might be not too bad, depends on the fabricator so choose him wisely and/or say a few Hail Mary's while you wait.
Btw, not sure how long your friend will agree to ride with you on tbe stock passenger pad, it's an instrument of torture. If you & her are keen, at least get it re upholstered. Won't improve its looks much but might help preserve your relationship.
J.