Convert mechanical relay to solid state relay
You want "form C" capability. Without a lot more information on what limitations you're willing to accept, it is difficult to answer this, but I'll take a crack at it. 

Since you are willing to accept an N-channel MOSFET with source grounded as your Normally Open contact, and the Common is therefore grounded, I'll assume you can accept the Normally closed also with the Common grounded. Keep in mind that both loads have to be from some positive voltage relative to ground). The below schematic requires a supply Vg for the N.C. MOSFET gate. 







simulate this circuit &ndash; Schematic created using CircuitLab

Another way is to simply buy an opto-isolated MOS relay that uses a depletion-MODE MOSFET for the N.C. contact. Using a depletion-mode MOSFET is about the only way to get a closed contact with no power applied.