Thanks for starting this conversation.
A popular and well-maintained OpenPGP GUI tool is Kleopatra (KDE) and it too doesn't handle notations, not even displaying them.
As I have mentioned on Matrix, as far as I am aware, not a single other project relies on OpenPGP notations (someone please prove me wrong!). To get philosophical: if an RFC has a feature but no one uses that feature, does that feature really exist?
Anyway, I sadly discovered the lack of support for notations after I started with the Keyoxide project, leading to the current state where the method works but no one uses it because there are no good user interfaces.
Software development libraries too tend to skip the notation chapter of the RFC. There are currently three helpful libs:
go-crypto v0.6.0 is less than a month old, so it probably supports notations now!
With all this in mind, it may be better to develop niche tools using those three libraries. Perhaps other projects will catch on and start using notations, and then the larger tools such as Seahorse and Kleopatra might join and support them.