Use ACLs to specify tcp_outgoing_address?

I think a really useful squid feature would be the ability to use ACLs
to select the outgoing IP address for initiated TCP connections (and
maybe UDP packets too).

For example, instead of:

# Can only select one of these at any particular time
tcp_outgoing_address 1.2.3.4
#tcp_outgoing_address 5.6.7.8

perhaps one could have something like:

tcp_outgoing_address 1.2.3.4
tcp_outgoing_address 5.6.7.8
# Use 1.2.3.4 in some situations
tcp_outgoing_address_access 1.2.3.4 deny acl ...
tcp_outgoing_address_access 1.2.3.4 allow acl ...
# Default to 5.6.7.8
tcp_outgoing_address_access 5.6.7.8 allow all

There are all sorts of reasons why one might like to use different
outgoing IP addresses based on some criteria available to Squid. This
includes the ability to:
(a) Route IP traffic differently (return traffic at least).
(b) Provide differential prioritization to IP traffic.
(c) Allow local or remote web servers to distinguish broad categories
    of user.

Now for some questions...

(1) Has anyone implemented such a feature already? (Or is anyone working
    on it, or planning to work on it?)

(2) Are there any reasons why this would be a bad thing to do, or a
    particularly tricky thing to code?

(3) If not, then does anyone have suggestions for what would be an
    appropriate syntax for specifying this in squid.conf? Of course
    there are other issues that would need to be decided on, such as
    what to do if no tcp_outgoing_address is selected. (Maybe there
    could be an implicit "tcp_outgoing_address_access allow all" on the
    first/last IP address, after all other tcp_outgoing_address_access
    directives.)

--
Chris Teakle
Network Development, ITS
The University of Qld