No plastic buckle would be worth the swap. Maybe a delrin buckle would hold up ok. If there is such a thing, it would probably cost double what the stainless one does and weight almost as much. Give it up.
The XS Scuba page Bob linked to has a Delrin weight belt buckle.
Diver's Supply has them for $3.95.
XS Scuba Standard Weight Buckle Black
If I was to consider one, it might be this one: XS Scuba Weight Accessories (top one), though there are numerous options here...
Truth be told, I think there are likely many other options in travel to shave weight besides a weight buckle.....
---------- Post added August 23rd, 2015 at 01:12 PM ----------
I am thinking of ordering one of the XS Scuba glass-filled nylon ones. $12 from LP.
XS Scuba Clear-Path Buckle AC311
I don't care about weight savings. I have been thinking about wanting to eliminate "shiny" from my rig. I have had a couple of occasions in the last month where I was in the water with a bunch of barracuda. A couple of times, I had one that seemed to be pacing/stalking me. Swimming exactly parallel and exactly the same speed as me for over a minute. Each time, I reached down slowly and covered my shiny stainless steel belt buckle and within 2 or 3 seconds, the barracuda swam away. Maybe it was coincidence. Maybe it was just my imagination. Maybe a barracuda would never actually strike at my big, shiny waist belt buckle. But, every time, it made me nervous and the reaction when I covered the buckle with my hand sure made it seem like that's what they were eyein'. If nothing else, switching to a black plastic buckle will make me feel better.
And, I'm with NetDoc on feeling the need for a big, beefy buckle. When I don my rig, I stand up before I even thread the belt through the buckle. Nothing is going anywhere, even with the waist strap completely undone. I think if it were to completely fail - the flap and pin completely fall out - I think I could put the tail of the waist belt through one of the inner tube pieces on my belt to hold it until I got out and replaced the buckle. And I'm pretty sure I could hang by the shoulder straps without a problem, too - i.e. waist belt was completely gone, BC fully inflated. The webbing through the bottom slots on my BP is not going to pull through and let me completely separate from my rig.