monitor message queue of a process

[Roberto Ostinelli]

dear list,

i would like to know if there's a native way to monitor and react on a
message queue of a process, without the need to modify the process
itself.

it's quite easy to get the message queue length using
erlang:process_info/2, however i would need this monitoring done not
in a polling-way [which would periodically check for this
information], but i would love to have some kind of a message when the
queue length is modified and satisfies some requisites.

i'm trying to figure out a way to provide some kind of generic
buffering technique to solve selective receives' snowball effect that
is being discussed so much lately.

ideas welcome.

r.