NetworkManager running under openrc does not appear to execute any scripts out of /etc/NetworkManager/dispatcher.d Unfortunately, that includes the script that's supposed to tell openrc that the network interfaces are configured and it can go on with higher level services. This prevents webservers, etc. from being started without manual intervention.
I've seen this on three independent machine configurations and NetworkManager versions 1.4.4-r1, 1.8.4, and 1.10.2 The only requirement seems to be that NetworkManager and OpenRC be configured to start network interfaces in the background. (I'm not actually sure if it can be forced to wait anymore, I haven't tried to do that in quite a while. From the looks of the init script the configuration might not matter, but I haven't tested it explicitly.)
I suspect this may be related to: https://bugs.gentoo.org/show_bug.cgi?id=477086 If the dispatcher has been split into a separate service, that service is not listed in /etc/init.d and does not appear to be started by /etc/init.d/NetworkManager
I've chased this about as far as my skill will allow. I'm not familiar enough with how NetworkManager works to be able to tell for certain if this is a result of the Gentoo configuration or if it's a bug in the NetworkManager program itself.