EDIT: I will try autofs next.

1. autofs s*cks.
2. /etc/rc.local must include sleep 30 / a ping check to the nfs storage, / mount only if successful, then exit. Gotta check on how to script it correctly. Last scripting-time is long ago : (

I inserted this right above the exit 0 entry in /etc/rc.local
Remember:
1. I won’t be responsible for any damages you cause from information given here
2. try your mount commands first, before you insert them
3. replace the ip with the ip of your nfs storage

# Set retry to the number of times you want the loop to repeat
RETRY=20

# As long as we have retries left...
while [ $RETRY -gt 0 ]; do
    # this will become true if 'ping' gets a response
    if ping -c 1 -W 1 192.168.0.1 > /dev/null ; then
        # insert all your mounts here
        mount -t nfs 192.168.0.1:/path/to/roms/snes /home/pi/RetroPie/roms/snes
        mount -t nfs 192.168.0.1:/path/to/roms/nes /home/pi/RetroPie/roms/nes
        echo "NFS mounted"
        RETRY=0
    # and if there's no response...
    else
        # set sleep higher if you want to wait more than 1sec between attempts
        sleep 1
        RETRY=$((RETRY - 1))
        if [ $RETRY -eq 0 ]; then
            echo "NFS Failed to mount, server sent no echo response"
        fi
    fi
done

Thanks to chrishsin for pointing me there.

Okay, so here we go.

What remains open is the part where I only mount one folder “roms/”.
But my mounts in .profile and rc.local totally went south.

Prerequisites:

• empty rom folders on the pi – because I mount the nfs folders into the existing rom folders
• a working nfs share on your IP (replace 192.168.2.1 with your nfs share)

What I now did was edit the file /usr/bin/emulationstation
and inserted the following code right after

#!/bin/bash

# Set retry to the number of times you want the loop to repeat
RETRY=20

# As long as we have retries left...
while [ $RETRY -gt 0 ]; do
    # this will become true if 'ping' gets a response
    if ping -c 1 -W 1 192.168.2.1 > /dev/null ; then
        # insert all your mounts here 
        sudo mount -t nfs 192.168.2.1:/volume1/shares/games/roms/snes /home/pi/RetroPie/roms/snes
        sudo mount -t nfs 192.168.2.1:/volume1/shares/games/roms/nes /home/pi/RetroPie/roms/nes
        RETRY=0
    # and if there's no response...
    else
        # set sleep higher if you want to wait more than 1sec between attempts
        sleep 1
        RETRY=$((RETRY - 1))
        if [ $RETRY -eq 0 ]; then
            echo "NFS Failed to mount, server sent no echo response, or check your network connectivity"
        fi
    fi
done

As long as your network connection is set up (I had minor problems with wifi) your mounts should come up and running. When you see the emus in emulationstation, everything should be fine.

Consider this one solved. But: I may reserve a post, for when I will be able to replace the multiple mountpoints by a single one for all emulators. (I might run into an issue due to too many mounts for that little box and my 54Mbit wifi)