I wouldn't imagine so, what the pensioners wear is a uniform, if you start with riflemen wearing black buttons you'll next hav eex RWF wanting to wear a flash on the backs of their collars, the Jocks will want to wear the kilt and the Irish will want green uniforms. Much as I can appreciate the thought behind the black buttons I don't really see how it's practical.
IIRC on the program about them they wear uniform as stated and buttons of the Royal Hospital Chelsea. They can wear the rank and medals/awards they were entitled to wear on discharge, plus any others after i.e MBE etc
Used to get some of them (Ex KRRC & RB) turn up regularly on remembrance day at the mess in Davies Street, I presume some still do. Never saw one wearing black buttons and am pretty sure they would have had it been permitted. The fellas I met were mafia through and through.