There are free scripts around that will allow you to store your swap file on a separate drive (for instance, search for SwapRelocator). These are invariably written in shell script and just modify some of the quirky startup procedure behavior on the Mac.
I'm sure you can modify it to mount a SPARSE AES-128 dmg (hdiutil mount <dmg file name>) and then use it as the swap file location. This should prevent anyone doing forensic attacks on the swap files.
I have not tried this myself. Obviously, there will be a performance it if your Mac has been idle for a while. A lot of people forget that the Mac is a Unix underneath. More and more utils are getting command line equivalents (Thank God).
Hope this helps,