I see what you mean by length. I just skimmed over the text originally as I don't have time for rather lame problems like this. I'd just add 3 bits of length to be part of the index, job done. 12KB lookup table instead of 4KB, assuming 0 is not a valid value (negate to avoid needing 0b11).
It's an interesting idea, but I don't see it being practical, even if the size of the table wasn't an issue.
The comparison here is:
((v ^ 0x303030) * 0x640a0100) >> (len << 3)
against:
table[(((v >> 12) | v) & 0xfff) | (len << 12)]
The former is 4 ops, the latter is 6 ops, so throughput wise, the former wins. Latency wise, it also wins, considering that L1 cache lookups are generally 3-5 cycles, whilst integer multiply is typically 3-4.