What you are talking about is called solera brewing; which in turn was borrowed from sherry brewing. The mad fermentationist
has done a whole series of articles on solera brewing (search his site for 'solera' to get the full list).
I would also recommend the mad fermentationists solera calculator
, a simple spreadsheet that will help you maximize the use of your barrel (i.e. dfetermining when/how much to withdraw in order to get the best product).
Its a cool project - I'm hoping to get a club-wide solera project going with my brewclub.
EDIT: the link to the spreadsheet on the blog appears to be dead. Here is the correct one: http://archive.org/details/SoleraAgingSpreadsheet