Played with this idea earlier, but ultimately steered away to make it easier for people to submit suggestions.

I'd rather have someone submit a suggestion with a wrong URL and have to figure out what they mean than prevent them from submitting by throwing an error. If the user can't figure out why the URL doesn't work, they may simply not submit their suggestion, which is worse than a broken link.

Also, we auto-fill the correct URL if someone clicks the suggest icon from a page - so that should help as the feature gets more filled out.