So you don't want to actually make a crawler but rather to allow the search engines to crawl your forum ?
First, about the phpBB bots handling, my advice is to barely not use it, it's ok for SID removing, but it's technically cloaking, so I would not go too far into configuring custom authorisations for bots.
Personally I set bots authorisation the same as guests, it's safer and usually there is no real need to go further.
About what you could do SEO phpBB, oh well, we've released several useful mods for this purpose here.
You could start playing with our SEO premod
to see all our mods at once without code editing.
From there you could either use it or use your selection of phpBB SEO mods. Mod rewrite and zero dupe are really useful, title optimization and dynamic meta tags are too