My question is, if your site requires a login to use, then Google Adsense will display security and password/login targetted adverts because the google crawler doesn't have a registration on the site.

Therfore, is it possible and legal (within the boundaries of the adsense program policies):

Obtain a list of IP's the Google crawler uses, and when it's detected the computer viewing the page is the google crawler, dont display the login page but keywords targetted at what should be displayed if you were logged in.

Or for that matter, is it ok to do that with keywords unrelated to your site completely (higher paying ones?)

You can check the user agent and redirect any crawler past your login page directly to your first page.

Routing them just to a keywords page, doesn't help as much as routing them to a page (e.g. home/section home page) that has links deeper into your site with related content on them.

Google doesn't seem to rely on meta-tag keywords as much as on content with keywords used in some proximity to other words (not necissarily keywords) found on well ranked similar sites - e.g. "ASP" used within 10 words of "code" differntiates between a page on active server pages and one on application service providers. Then if they see the word "example" on the page, they might rank you higer.

I would think that if you had add-sense links that were not related to what brings visitors to your site, they would be less inclined to click on them?

Do you know if this falls within the boundaries of the adsense program policies?  I can't see anything forbidding it.  I just want to rid of the password and login adverts which don't pay well and are boring.

It's been a while since I did anything with addwords, but I can remember submitting more than one page of a site and getting different add words for each. Also asked that specific adds be excluded, which they did.

Try putting in the user agent redirect for the bot, then submitt the page they are redirected to for addwords.