lucy24 - 2:57 pm on Jan 26, 2013 (gmt 0)

I can put an htaccess in each of the subdomains

There is no point to this, because you will be intercepting the requests before they get to the subdomain. Otherwise you'd have to make thirty separate htaccess files, each saying exactly the same thing: first a rule to redirect ^(index\.html)?$ to the new place, and then a second rule to redirect everything else to the main domain. And you would still need something complicated in the top-level htaccess to do the regular index-and-canonicalization redirects for everything that isn't a subdomain.

The part I didn't get is whether each subdomain's top-level index page gets a different /folder/ redirect, or whether they all go to the same place.

Thirty is an awkward number. It's in the grey area where you can either do it in htaccess or send everything to a php script, but it isn't big enough or small enough that you absolutely have to do one or the other.

Do the subdomains have subdirectories? That is, anything in the form
subdomain.example.com/directory/index.html
subdomain.example.com/directory/subdirectory/pagename.html

as opposed to simply
subdomain.example.com/pagename.html

If so, where will they go?