How can I automount a shared network drive for Time Machine backups?
I think I can get you started, but using terminal commands, which you said you'd prefer to avoid. Apologies, but it might give you a starting point. All of the following you could put in a bash script and run as a login item. 

You'll need to first share out the target drive(s) (MyBook in the examples below) using file sharing (System Preferences > Sharing > check the File Sharing box, add the drives you need and set appropriate permissions - I just use Everyone to keep it simple).

You can mount a remote machine and/or its associated drives using mount like so:

Make a directory as the mount point (obviously call test whatever you want):

mkdir /Volumes/test


Now mount the remote drive. I'm assuming this is already connected to your iMac and appears when you connect to your iMac using Finder:

mount -t afp afp://&lt;your mac's name&gt;/&lt;drive name to mount&gt; /Volumes/test


On my remote Mac Mini, to connect to my MyBook attached to it via USB, that would be:

mount -t afp afp://bobs-mac-mini/MyBook /Volumes/test


If you need to authenticate (I got error -5000 when trying to mount my home folder), you can also do this using

mount -t afp afp://&lt;username&gt;:&lt;password&gt;@&lt;your mac's name&gt;/&lt;drive name to mount&gt; /Volumes/test


However, the password would unfortunately have to be in the clear. So again, this might be:

mount -t afp afp://binarybob:password123@bobs-mac-mini/MyBook /Volumes/test


You can also connect to your home folder using the above method, just by replacing MyBook with the name of your home folder.

The drive you mounted should now appear in the finder and you should be able to use it like any other locally mounted drive. When you're finished, you can do:

umount /Volumes/test


to remove it. BTW, if you're not an administrator, you might need to add sudo in front of each command and type an administrator password.