VirtualLock() to stop private keys being written to disk?
Unfortunately not. The VirtualLock() function in the Windows API doesn't do a proper job: it may prevent small pieces of a process's memory from being paged to disk while the process is running, but it doesn't stop the process's memory as a whole from being swapped completely out to disk when the process is long-term inactive. And Pageant spends most of its time inactive.