Those of you paying attention to d-d-l would have noticed that Seahorse was proposed for inclusion in GNOME 2.18 recently. One of the concerns brought up was that it wasn't integrated enough and to that effect an Epiphany plugin should be created.
Such a plugin now exists! Jean-François Rameau of #epiphany did the majority of the work by creating a framework for adding items to the context menu and handling mozembed calls to change the text in entry fields. After struggling a bit with build magic and sprinkling with libcryptui and dbus calls I checked it in this afternoon.
More work needs to be done but it's usable. Future work includes decrypting, verifying and signing the contents of text fields. At the moment the plugin works with the additional comments field on b.g.o but doesn't handle the message body field on GMail.
I've created an Epiphany component under Seahorse on b.g.o. Please file bugs there.