Whatever the normal visitor can see in your site is what the Google bot will see too. If the normal visitor gets redirected to the MOP (Membership Options Page), so will the bot.
So you can't have the bot index the content of a protected page but not show it to the normal visitor. Well, you could but this is frown upon by Google and it could penalize you.
You could show the content for a moment and then redirect, but this would be easy to side-step. If you do it with JavaScript, you just disable JS on the page and you remain there. If you use an HTML redirection, I could save the page before being redirected.
One thing you could do is show a part of the content as a teaser and the rest only to paying users. Instead of the metabox to apply the access restriction that would redirect, you can use conditionals in the protected pages.
WP Admin -> s2Member -> API / Scripting -> Simple/Shortcode ConditionalsUsing a filter to edit the content of the post/page, you could probably create a template for them that applies the conditional to all posts/pages instead of having to edit each one.
I hope this helps.