That's a really good idea, and I'm thinking about waymarks already. Here in the UK there are lots of detailed lists of the top hills/mountains and figuring out the tallest in various 100 mile circles could be a nice little project for a rainy day.
Would you ask for online evidence to back up the claim? Or would you allow a "challenge" eg, "I've no reason to believe there's a taller statue for 100 miles than that of General Oddbod atop his 50ft column in Oddville. Disprove this if you can".
Would you require the waymarker to actually GO to the top? Obviously not for statues (because that would be impractical) but for buildings it would be a real issue (you can't get to the top of most tall buildings in London for security reasons). But for hills, it would be a bit lazy to post a waymark in the carpark at the trail head and say "I couldn't be bothered walking all the way up there".