The newest version of KeePass, 2.23, was released by upstream on 2013-07-20. It should probably be put into Portage. I could add it to my overlay, but I'd rather have the main tree version bumped, so everyone can benefit.
Which version of libpng do you have? I have noticed this bump, but avoided pushing it because of bug #472764.
I have libpng versions 1.2.50 and 1.5.15, but if bumping keepass would break the package for those with other versions, I can understand why it hasn't yet been done.
Ah yeah, stable libpng still works fine, but 1.6 breaks keepass/mono/something.
+ 23 Oct 2013; Tom Wijsman <TomWij@gentoo.org> +keepass-2.23.ebuild, + metadata.xml: + Version bump to 2.23, fixes bug #472764 and bug #486936. Still has an + unhandled exception when closing the window that still needs to be resolved, + see bug #489120.