This is very good question and I'm looking forward to all good responses. I'd like to add my two cents though.

Personally I just generate the page in utf-8, put into the content-type header that charset is utf-8 and add the accept-charset attribute to the form tag and hope that the client does the right thing:

<form [...] accept-charset="utf-8">
[...]
</form>
[download]

This seems to work fine and the content from the clients does come in utf-8.