Beginning with bitcoind 0.6 (not yet released), you will be able to control the creation of coinbase content in blocks you mine. This is split into two parts:
The coinbaser is an external program which tells bitcoind how to distribute generated funds; it is called whenever bitcoind wants to build a new coinbase transaction.
Usage (bitcoind 0.6+)
To setup a coinbaser, you need to pass a command to bitcoind using the -coinbaser option. This command will be executed by your system's default shell. If the string '%d' is present in the command line, it will be replaced with the total amount of funds available for distribution (in satoshis). For example:
bitcoind -coinbaser='/home/user/coinbaser.pl %d'
You can also have bitcoind connect to a TCP socket. In this case, the socket will be sent "total: " followed by the funds available and two newlines. For example:
The coinbaser has a simple line-based format for standard output. The first line should contain the number of additional destinations for generated funds. For each destination, there should be two lines: first, the amount of funds to redirect (in satoshis); second, the bitcoin address to direct them to.
If there is any error in this format, a reason will be logged to debug.log and bitcoind will abort customization and revert to its default behaviour of taking the full reward+fees for itself. Note that distributing all the available funds is undefined behaviour; remaining funds are generated to the original destination, and treated in the usual way. This can, however, safely be as low as 1 satoshi left not redirected.
setworkaux is a new JSON-RPC call to add arbitrary data to your coinbase transaction's "scriptSig". Once added, it will be put in all new work (see getwork) until explicitly replaced or removed. The first parameter is required, and must be an arbitrary identifier for the data chunk. The second parameter is optional; if present, it must be a hexadecimal-encoded string of the data to be added (or replace older data under the same id). If the second parameter is not provided, it will remove the id from future work. There is a limit of 100 octets total in the coinbase; setworkaux will fail if you try to add more than it can safely insert.