The titanium collar clamps are less available because of cost to manufacture a very small demand, and lastly.... for those counting gram's, they weigh more than the svelte aluminum offerings.

Haven't seen a clear photo of the seat tube (ref. the frame seat tube) but suggest drilling a relief hole at the end of the expansion slot. Also, for that smaller diameter titanium tubing, the seat post fitted to the seat tube should be a slightly tighter tolerance than just slip fit. Focus on this and then the pinch clamp is less stressed, plus allows one to seek the lighter weight, elegant type without concern.

Ample selection of 28.6 seat post collar clamps on ebay. The Thomson is nice and from a long time supplier to the aerospace industry, consider its reliable. Its pricey and double cost many other offerings.

I wouldn't be hung up on seeking a titanium clamp as there's many aluminum, light in weight and attractive designs. Try Omni Racer as example.

In titanium, you might be able to reach out to asian supplier like J&L. They're pretty decent to correspond with. (side note: I had a new seat post from them but noticed a flawed rail clamp. They agreed and promptly replaced.)