My only experience of creating items in the library is of being invited. We can make good quality avatars swiftly. I would imagine it depends on what they are looking to have in there. If they are looking for the items that you make and they are good quality chances are you might be invited?
The rest you'd have to ask LL as it's not my decision.
As to marketing, there are nice elements and negative elements involved in creating for the library, not least days like today when I had to deal with overwhelming IMs, notes and even accusations of trying to take peoples Lindens. I'd have swapped my day with you happily! However in spite of that I have been in SL a long time and I am proud to be asked to make items for the library.