no 720pn mode does not offshoot to the drive. I have heard they are working on it.. but i think its more of a camera limitation rather than a firestore limitation.
Using the FS-100 is just like using the P2 card. Record in 720p24 mode, use variable frame rates. Then when I import into Final Cut Pro, I can use it's Frame Rate Converter tool to make it all just like I shot it in 24pN.
BTW, do any NLE's at this time support the 24pN format natively? I don't think so, but, I'm going to test it out!