someone said somewhere that you can actually subscribe to yahoo mailing lists by sending an email to group-subscribe@yahoo, they just don't tell you this

That was my thought, too.

I don't know if this true.

ISTR I did this at least once (and later ran into funny situations when I tried to access that group via the web since the subscription was in a sort of funny state since it was tied to my email address but not my Yahoo ID) - but I wouldn't be surprised if it depends on the group (i.e. some might let you subscribe by email and others might not).