modifying config on the fly

As far as lowering the active connections. You could force offline the pool member to clear out the connections, but this will kill all connections. Merely disabling the pool member will not clear out active connections if I remember correctly.

Alternately in 10.2.4 you could list connections and selectively kill sessions or a number of sessions down to the new limit. This does require cli access though. "b conn help" will give you the syntax for deleting connections.

As Mike pointed out, those don't help in a situation where someone wanting to adjust the connections doesn't have GUI or CLI access. There aren't a whole lot of options. My recommendation would be to lower the connection limit to the minimum amount supported during a bad period on the server.

Alternately, the F5 might be able to help out the back-end server. The one-connect profile, and caching might be useful. Lowering the tcp-timeout might be useful in clearing out old connections more quickly if the client doesn't do it properly. If you lower the tcp-timeout below legitimate traffic you could worsen the issue.