1. The DS cannot use virtual memory. It has no MMU.
2. Even if the DS could use virtual memory, patching the Opera browser to use that instead of actual RAM would probably be fairly difficult.
3. Even if it was possible, it would quickly wear out your microSD card.
There are ways to hack the Opera browser to use forms of slot-2 RAM other than the official RAM pack, but you still have to have SOME form of RAM there in order for it to work at all.