Conform pattern to arbitrary shape?

There are some things that are hard to do, difficult to do and just do not work well without a lot of tweaking to do in FF. One of those things that fall into all of those categories is making a pattern follow a selection. Making a pattern do as you suggested would have to be on a case by case basis. There are some filters in the library that do follow a certain shape but those were created with that in mind. There are ways to bend and shape the patterns themselves but most of the times they do not look correct or do not do what you want them to do. Throwing in a selection really makes it more difficult for tweaking and controlling the final result.

Here is a filter by Uberzev that turns an image into a shape. SHAPE FILTER