I'm rather confused by your question... you want to offer people the opportunity to add your site (which isn't mobile friendly) to their home screens?
It's been well established in other threads that replicating browser behaviour (such as print, text resize or bookmarking) unnecessarily is obtrusive and counter-intuitive to web accessibility, but the idea that you're trying to encourage people to "home screen" (indicating a web app) your site (which isn't) against the convention is like pouring gasoline on a fire (to Apple users). In regards to Google, their web apps simply attempt to use the HTTP_HEADER to recognise the iDevice and thereby serve the message (as their apps are optimized for the experience). As for your particular issue, you should not be actively trying to encourage the user to add your site to the home screen (rather than bookmarking it) when the experience does not match the intended outcome. Perhaps you won't agree with me on this issue but you're undermining the purpose behind the home screen functionality - and anything which breaks the convention (especially on Apple devices) will not be well received.
PS: If the end user wants to add your site to the home screen, they can do it within the browser in two clicks - you're trying to over-engineer your experience.