<Danyal to get very high humidity the soil will be damp/wet anyway...>
not really, in my 20g tall treefrog viv the soil is partially damp but the humidity is only slightly above house levels(screen top) but the 25g show/long had very high humidity but the soil is very dry (glass top)
<i guess a viv the soil would be mostly covered in moss so it would have to be moist. would it really be much better/easier to do amphibians?>
amphibians would really be the best candidate for a moist/high humidity viv. you can have dry moss but it won't be alive and growing wich isn't really a big downside. there are several species of very insteresting and bold amphibians. or maybe some geckos, i know some dart frog keepers will keep day geckos(?) in with their frogs.