This is a slightly different (and easier) procedure to the sythvegeta's method to install demos not available in the PSN store:
0- I assume that you have the PS3.Proxy.Server already configured
1- Go to the PSN store and start the download of a large demo (it must be larger than the one to be installed). IE. Ratchet or Bourne
2- Select to send the download the background and exit from the PSN store.
3- Go to Download Manager in the XMB and pause the download.
4- Copy to the clipboard the URL of the demo (the one of step 1) that appears in the Log tab of the PS3.Proxy.Server
5- Go to the Replace Files tab, press the Add button and paste the URL copied in the step 4 in the left field and select the local PKG file to be installed in the right field.
6- Restart the download in the Download Manager found in the XMB. If you select to show the status, it should display the actual size of the demo stored in your PC.
7- After the download is completed, select to install the bubble found under Game in the XMB.
And how should I install the Resistance 2 Private Beta? It has 1711MB and there is no bigger demo than 1650MB with Bourne
But in the Thread where the links are someone has done it.