This is an interesting question. At least to the extent that such external investments are considered to have extremely low credit risk, in theory the bitCurrency yield should be linked at least partially to this external rate. If shorts failed to build it into their offers, there would be an arbitrage opportunity, because arbitragers could short the bitCurrency and go long the fiat Currency (from BTS), investing this into a bank deposit or short term bill, thus having a neutral position in which they earn the interest rate differential. Also there would be less demand as users would compare bitCurrency yield unfavourably against external alternatives. So in theory, arbitrage should ensure this is built into the bitAsset yield. In practice however, there may be significant cost of the arbitrage that hinders this process (e.g. not everyone can invest roubles into their local bank or bonds).
For longer holding periods, where the interest rate differential is likely to be higher and possibly outweigh the costs of arbitrage, the free market for the bitBond yield should reflect these external rates better. In principle, apart from cost, I think a free bond market should be able to handle this.