[Exim] filter on a domainlist-router

hallo,

can anyone point me to the right direction how to relaize am mechanism
to send to some domains only the first 512k of the bessage (or exactly
message body)

1.) have i write some code myself within a filter?

if $h_to: contains "domain1" or
$h_cc: contains "domain1" or
$h_to: contains "domain2" or
$h_cc: contains "domain2" .... (which is not very nice)
then
pipe "/bin/mystripmail 512k"
endif

2.) or is there any other way within a domainlist router.
(i did not find anything in the documentation)

robert