It turns out that you cannot patch network installation points directly.
Not surprising really. Why should Microsoft make software deployment manageable out-of-the-box?
Nevertheless, I found a useful article at WinnetMag.com.
You can patch the network installation point directly, by using the following command:
msiexec /a < original .msi and localtion > /p < .msp file and location >.
However, you then need to redeploy the application afterwards. This can be done from within group policy.