Other users have reported issues booting from USB on various current Dell platforms. Strictly speaking, most of the new stuff isn't on the FreeBSD hardware support list (and therefore not the FreeNAS support list), but there is no reason it shouldn't work.
See the following thread (particularly the tail end) for info on getting USB boot working:
I found your thread because I have a customer that is looking tu run FreeNAS on an r720XD. Please let me know how your project goes and if you run into any other issues.