No easy way to join discussion in existing thread when not subscribed

The flat web interface does a reasonable job of letting people
follow discussion without getting too much mail. Once you find
a discussion you'd like to participate in though, there's no
easy way to make your first post to thread, because threading
is based on the "In-Reply-To:" or other some mail headers. If
you're not subscribed, you can't "Reply", and so your email
doesn't end up linked to the thread in the flat view (I'm
guessing most people use that, if not their mail client).

So far, I've been downloading the mbox, importing it to my mail client
and then replying and it's quite cumbersome.

I've read elsewhere that a possible solution is to provide a
mail address associated with a message (displayed on the web interface)
that routes your mail message to the thread. I'm not sure
if majordomo supports this, but if or another solution
is possible, it would be a welcome improvement.

Regards,
Amir