I would say "Not Safe" just because Nail Coating - I presume this is a transparent Nail varnish - contains solvents (such as acetone, butyl acetate or ethyl acetate) which evaporate leaving the hard nitrocellulose coating. The solvents may attack, embrittle, discolour other components of the phone. I'm sure in the user-manual it says not to use solvents?
if you somehow successfully painted just the chamfer, over time the hard coating would chip-away, become a bit messy, maybe attract fluff or bacteria. Then you'd have to strip it and this involves more volatile solvents and potentially bad chemistry?