@errhead This is a client side check, but I need info to be available via federation.
Anyway, I added support for "lightning address" and "lud16" fields and my server now shows donation buttons if these fields are present:
https://mitra.social/post/018b6102-3d77-a88b-64b1-64d8c4578bc4
Ideally there should be an agreed-upon field name (e.g. $LN), just like $BTC, $XMR and $ETH (which are already used in Fediverse).