How do I arrange for all incoming email for *@some.domain to go into one pop3 mail account? The customer doesn't want to add a list of specific local parts to the system.

Set up a special transport that writes to the mailbox like this:

special_transport:
  driver = appendfile
  file = /pop/mailbox
  envelope_to_add
  return_path_add
  delivery_date_add
  user = exim

The file will be written as the user exim. Then arrange to route all mail for that domain to that transport, with a router like this:

special_router:
  driver = accept
  domains = some.domain
  transport = special_transport