I have been thinking about how this feature can be implemented in the current UI.

One possibility is to add a tab along side the project, user and customer tabs. In this tab you can choose which items to use for the filtering (or searching). A good reason for putting it here is that these other tabs are doing the same sort of thing - filter what you look at.

Since there are so many fields, especially if there are custom fields, then we might need to choose which ones we want to use. You might allow this by having a popup check box menu, similar to the one used to say which columns are visible.

Once you have selected your fields you then can do your filter/search. You select which fields and items in the fields to search /filter with.

Ideally, which fields are used to filter could be saved with the view, since you might want to search/filter with different values in the same field often.