I can confirm that running the following does work (ie, using the same interface index provided by ioctl to get the name as the interface index):
{:ok, sock} = :socket.open(29, :raw, 1)
# confirm ifindex and ifname
{:ok, ifindex} = :socket.ioctl(sock, :gifindex, 'can0') # returns {:ok, 2}
:socket.ioctl(sock, :gifname, ifindex) = {:ok, ~c"can0"}
# using index from ioctl doesn't work
addr = <<29::size(16)-little, 0::size(16)-little, ifindex::size(32)-little, 0::size(32), 0::size(32), 0::size(64)>>
:socket.bind(sock, %{:family => 29, :addr => addr}) = {:error, :enodev}
# hard code to index 0 works
ifindex = 0
addr = <<29::size(16)-little, 0::size(16)-little, ifindex::size(32)-little, 0::size(32), 0::size(32), 0::size(64)>>
:socket.bind(sock, %{:family => 29, :addr => addr}) = :ok
I can give this a go! But I think this part seems to be working at the moment, I think I was more referring to the interface indices are probably not the same since the network interfaces might be configured in a different order for Nerves vs Raspbian. But let me try your suggestion 
