I believe what you are looking for is being discussed in this other thread:viewtopic.php?f=32&t=514
Unfortunately there currently isn't any way to have one item reference another in MyStuff2. Your use case would be perfect for the feature being discussed.
The only possible suggestion for now would be to setup your patterns and materials as lookup lists instead of categories with items. Then you could have a project category with attributes that reference the pattern and material lookup lists.
You could still have the materials and patterns categories in addition to the lookup lists. There just wouldn't be any link between them other than you using the same names.