A custom metal frame would make it a totally different gun and one need to find someone who can machine it out of a chunk of stainless steel for example. That would require a 3D scanner that can transfer the data to a CNC machine. If that would be possible it would also pretty dang expensive. Who ever does that job would also need to have a manufacturer FFL if that would be done for anyone else than the person who actually does the work. There might also be legal issues coming up with S&W when you copy their design.

If you want a full metal gun - add another gun to the collection.
Also one might want to have the slide milled out in certain spots to reduce weight, or put a reflex sight on. I've seen pictures of other pistols like Glock's or M&P's that got that treatment. But then again, the SD series is a $300 gun and customization of those kind would probably cost more than the gun itself.
A way easier and cheaper way to customize your SD would be the ceracote route which would gave you the option of different colors on the slide and frame.
The only thing that I did to my SD9VE was putting the Apex spring kit and trigger in. That reduced the trigger pull to about 6 lbs., reduced the take up and eliminated the over travel. Also have the +3 mag extension on one of my mags which gives me 19 rounds now. I also put a Hogue slip on grip on it to increase the grip circumfence and enhance the fit to my hands.