Russia is asking for payment in RMB, and
if China does the swap, Bangladesh will surely be allowed to use it to trade with Russia.
China will NOT swap RMB with Bangladesh to cover Bangladesh and Russia trade. As I have said currency swap is NOT free flowing of currency like Euro and Dollar, it is ONLY to cover bilateral trade, the amount is only enough and delibrately limited to bilateral trade, otherwise it is not swap anymore. What you suggested is free flowing of RMB like dollar.
In a swap for every RMB given to Bangladesh, China will take in relevant amount of Taka. Both will hold these money for a period and return at the end. To give Bangladesh the extra amount of RMB for trade with Russia, China will take in more Taka which China has no use. Why should China do that? China swap with Bangladesh is to encourage Chinese export. What you suggested is to benifit Bangladesh and Russia with China sacrificing (holding large amount of useless Taka doing nothing for some years). If Bangladesh has to use RMB to pay Russia, it can borrow RMB as a loan then pay back with interest.
Always remember, China is helping, not running cherity.