First off, only a fraction of the CBRs reserves are held as US dollar denominated holdings. Most central banks hold a variety of assets (currencies and gold) to diversify currency risk (and in the case of the CBR sanctions risk)
Secondly, even most of the USD holding are not held as actual deposits at the Fed. For one thing, Fed deposits do not pay interest. There are also relatively illiquid (even if we don't consider sanctions.)
Instead dollar holdings are held as US Treasury bonds. The US Treasury bond market is the largest financial market in the world and extremely liquid. And treasury bonds do pay a small interest. A US treasury bond is the closest thing to being an actual dollar without actually being a currency.
These bonds are held in the bond market accounts of large US and foreign banks. These accounts are frozen too.
However, because US treasury bonds need not be held directly, the vast bulk of the CBRs dollar denominated holdings are probably not directly held by it and so harder to trace. Indeed, only the CBR will know exactly how much USD denominated holdings it actually has.