@domcleal i can put a method for managed_ip in lib/fog_extensions/aws/server.rb, so the value is now submitted there from the form.

I've also put a helper method for public/private ip in the EC2 class, but having a bit of a brain block on how to access the value in the Server module from the EC2 class. what is the best way to reference it? 
I can see that the managed_ip value goes into compute_attributes, but how to access that in the EC2 class?

The form:
<%= selectable_f f, :managed_ip, {_("Public")=>'public', _("Private")=>'private'}, {}, { :label => _("Managed IP") } %>
produces: 
<select id="host_compute_attributes_managed_ip" name="host[compute_attributes][managed_ip]">

EC2 class:
def managed_ip
  compute_attributes[:managed_ip] == 'private' ? :private_ip_address : :public_ip_address
end