Sorry for just telling the idea and not to offering to do it myself. But I think this is better than nothing :)
The refactoring browser integration into OB provides that functionality.
1. Left-click on an element of the package you want to search. 2. Click on 'Open Environment | Package'. 3. The opened yellow browser scopes all Refactoring Browser actions to what you see in this browser. 4. Either use 'Open | Search Code' (for parse tree searches) or 'Open Environment | Find Sources' (for full text regexp search).
Lukas