The index maintenance for transactions follows the same model as non transactional tables - coprocessor based on data table updates that looks up previous row value to perform maintenance. This is necessary for non transactional tables to ensure the rows are locked so that a consistent view may be obtained. However, for transactional tables, the time stamp oracle ensures uniqueness of time stamps (via transaction IDs) and the filtering handles a scan seeing the "true" last committed value for a row. Thus, there's no hard dependency to perform this on the server side.
Moving the index maintenance to the client side would prevent any RS->RS RPC calls (which have proved to be troublesome for HBase). It would require returning more data to the client (i.e. the prior row value), but this seems like a reasonable tradeoff.