Shoutbox

I don't think theres any real way to detect if a contact is using Plus! except by sending a ping request, which means you'd need to wait for a response before deciding whether to split the message or tell you its over the limit.  I'm also not too sure how much control Plus actually has over the "Contact says" and it might be extra hassle for Patchou!

Besides I think the text file idea sounds absolutely great.  Perhaps a similar idea could also be used for files - if you try to send a file and it's of a dodgy extension (I get it frequently despite disabling all the options) maybe it could automatically rename it to (and from on other end)  Filename.ext.pluszip or something to make it acceptable.