Does anyone make a split seat clamp?
DMR make hinged clamps for mounting chain devices and bottle cages. 3 different sizes and there is a 31.8mm which should be perfect. Easy job to retro-fit an existing QR lever and bolt I reckon?

Part Number: DMR-CLP-286

 

Alternatively here is something from a different application that would probably work. You might be able to contact the company for a 'sample' :)

URL: Staff-Lok Collar


(it's steel)