Good question. Don't know about the '65, but I can tell you according to my original '64 accessories book seat belts and seat belt retractor (code 624) AND the standard belts with out retractors (code 411) were available in harmonizing colors. This was a dealer installed accessory for LeMans, Tempest and Custom. Doesn't mention any belts for the rear, but I bet it could have been done. I'm assuming this was unchanged for '65.
Going further back, the front AND rear belts were offered in the Buicks in 1958, also as a Buick GM engineer approved accessory. I have one of those original booklets as well.
1958 Buick Special 2 Door Riviera
1964 Pontiac GTO Post Coupe
2012 Shelby GT500 convertible