It would be possible to fake such a cache file, but I doubt that it will work as you want it to.
Specifically, I suspect the RBT tool uses its cache as a C8 lookup before attempting the rainbow table search.
I very much doubt that it grabs PUSI payloads & checks for the expected initial bytes 00, 00, 01 after trial decryption with key after key, which is what BF programs do.
In short, it might work, but only for those feeds where you've managed to find an often repeated payload of known plaintext ie those bearing a C8.
And yes, the structure of the cache file is simply:
<8-byte C8><6-byte SW>