We have a huge solution with nearly 100 projects. The OFIW dialog soon becomes less useful due to the number of hits.
The OFIW is great, but often I'm only interested in files in the current project (I'd say 90% of the time). I think there should be a checkbox in the OFIW dialog that would make it an OFIP (Open File in Project) dialog. When checked, only files from the currently selected project would be displayed.
Use the context menu in the Open File in Solution dialog to display the edit control for the persistent filter. Hide the edit control after adding the text that you want to always persist. The text affects filtering whether or not the control is visible.