Let's get detailed. The chassis-the metal insert in the grip frame-is the firearm. The serial number must be formed on that item in a manner consistent with federal regulations and visible from the outside. IIRC, electro pen engraving isn't acceptable. I don't know off the top of my head if it's even possible to do so without destroying the chassis or frame. On the 1.0 the chassis appears to be cast into the frame.
At least one other maker puts the observation window in the bottom of the rail. Generally out of sight, out of mind, but acceptable under the regs.
I expect a couple of others on this thread have had similar discussions with folks. Just because you want to do something doesn't mean it's safe, possible, practical, legal or all of the above. And, BTW, I expect you can kiss factory warranty service good bye. So far as the legal part, I'd suggest you contact your local BATFE field office.
