This is not going to work.
If you create a common currency between all 5, you either adopting the same common currency (like EU and Euro) or you have to be able to freely change into that currency from your local currency like a token.
Would China want to bend that far backward to accommodate the other 4? Which have less than half the economic productivities than China? So do you supposed China just going to ignore their own advantage and go equal between BRICS on its currency??
Or how about each BRICS country hold their own currency and convert that to token? Then how does it work when Yuan and Rouble are the most regulated currencies on earth?
It won't work either way, you may as well ask BRICS to adopt Yuan as Official currency, but then what happened to USD will happen to RMB in a smaller scale, it will devalue RMB.
The mechanism for this common tradable currency / token can be worked out if there is political will or Economic need.
The real question to be asked is if there is indeed any intent or need.
The BRICS currency reserve will just be the acceptance of the Yuan for trade.
Nope, in which case we would rather have the USD.