The current nautilus-sendto ebuild depends on mail-client/sylpheed with sylpheed use flag, but this feature works with sylpheed-claws too. So I think that a "||" dep can be added. In the future sylpheed-claws will be replaced by claws-mail, but I tested only sylpheed-claws.
Created attachment 104205 [details] nautilus-sendto-0.8.ebuild.diff Diff for nautilus-sendto-0.8.ebuild that fix this. It works on my system.
As an aside, adding Claws-Mail support to nautilus-sendto is ready, waiting for a GNOME commiter to apply my patch there: http://bugzilla.gnome.org/show_bug.cgi?id=386767
Sorry about the wait, but this patch is now in portage, rejoice!