I'm building a Farrow rifle using the castings of Rodney Storie.

I have read somewhere that it is possible to work with the ATF to have a serial number placed on a firearm produced with 3-D printing.  I've been all over the ATF website but it's pretty confusing.

The ATF doesn't care if you build a single shot like this and don't even bother them regarding a serial number.  BUT!!! generate one that is relevant to you, stamp it on the action, use the last digit or two of this number and stamp the other major parts with that, including the wood.  If you happen to build several especially of the same make you will appreciate it.  The other reason for your number, if some low-life steals it, then there's a number you can give to authorities so they can keep a look out for it.  It does work out in some cases.
If your firearm goes to an engraver or gunsmith later on, there's a number for them to log and operate with.
Original Farrows had their #'s stamped in the face of the action, I stamp mine there but it's easily identified as one of my builds rather than Milton's.