Customize Your Open File Dialog from the Microsoft MSDN Magazine has a lot of information on the dialog. I haven't had chance to read it all, but this caught my eye:
A Custom Places Bar
...
You'll need a REG_SZ entry if the name of the folder is an absolute or relative path. You need to use the folder-specific number if you want to target a special folder (see Figure 6 for a list). In this case, a REG_DWORD entry is needed.
Figure 6
Folder IDs
ID Folder
0 Desktop
2 Programs folder on Start menu
3 Control Panel
4 Printers
5 My Documents
6 Favorites
7 Startup folder on Start menu
8 Recent Files
9 Send To
10 Recycle Bin
12 Start menu
17 My Computer
18 My Network Places
20 Fonts
I've missed a whole load of stuff out (because it's a very long article), but it looks like you can set the ID value to 18 to get your network places. However, as @Nelson points out this might part looks like it's adding an entry to the bar, so double check it before using. As I said before the post I've linked to contains a lot of information, so what you need may well be buried somewhere in it.