High temperature flexible filament
Ok, so to answer the primary question: What flexible filament will operate consistently at 250°C?
Man... this is a tough one. Some filaments, like PEEK and ULTEM 1010 can operate up in the 200c range.... but they're not flexible at all.
Silicon might be able to work, but you're still pushing boundaries.
Now.... I'm lucky to be in a 3D Printing company.... and we're testing a super-high-temp flexible material, very similar in temperatures to ULTEM. I'll definitely check back and let you know how it goes, but....
Honestly, that's so hot, readily available thermoplastics may not be an option unless you're in Aerospace with an unlimited budget..... which, based on the requirements, would make sense, lol!
I'd say the most readily available way to get this done would be 3D Printing a mold, in which to put your silicone, and bam -- you've got the part.