Is it possible to figure whenever the cbmem buffer supports rollover or
not, so a updated seabios version can work correctly with both old and
new coreboot?

Not explicitly, but it should still work fine with both versions. The old coreboot can only either not fill up the buffer or fill up the buffer. If it fills up the buffer I can make SeaBIOS detect that (it would look different from rollover with the new code) and just not append anything else, just as it would do right now. If it did not fill the buffer, SeaBIOS can just continue appending there and roll over if it needs to (the buffer format stays 100% the same before the first roll over). Then on the next boot, an old version of coreboot will just unconditionally reinitialize the buffer anyway.