Try using the other chat application called gaim. I haven't tried sending files with it but to me it looks more promising than kopete.
as you may know I am the one that posted the original question on this thread so I am experimenting with various alternatives. So far I have seen that the send file option is available for MSN and AOL instant messaging services using gaim.
for more info consult the following link:
http://gaim.sourceforge.net/faq.php
this page will answer any of the questions that you might have about gaim.
I hope this helps, I will let you know if I find a better alternative because I too am trying to solve the same problem of sending files etc. through cross platform messaging
good luck!