Unfortunately not. There's no way to 'cache' filesystem structures with Quicksilver. It asks the disk for the current files/folders every time you you → or ←.
I'm not really sure if there would be a way to do what you say, since it would break many other locations.
The only solution I can think of is to create a text file with the paths to your most 'important' files (one path on each line) in it, then when you want to browse to the network disk, just find the text file and hit →
You will need to install the Text Maniplulation Plugin to be able to → into text files.
It could look something like this:
/Volumes/My Volume/Important1
/Volumes/My Volume/Not Important/Important2
/Volumes/My Volume/Images/Photoshop/My Images
etc. etc.
Patrick