Actually i don't think it can hurt your google rankings. There is no way googlebot tries to simulate zango toolbar (by changing "USER AGENT") and see if you will redirect him (googlebot cannot access your .htaccess). It's the same like splitting users using IE and Firefox browsers to different layout pages. In the thread you provided people discuss a warning page (extra step), not a redirection of infected users (totally different issue)
About TGP - well it will kill 2% of your incoming traffic for sure, and your script will not see them. But if other TGP's are sending you zango infected traffic - it's the same like they were sending you hitbot army who doesn't buy anyway.
All info is IMHO
