I have tried some Nancy Holder and was likewise unimpressed. If I had any idea where to start looking at tie-in writing, I would do so. :-D (I imagine having original fiction under contract might help, proof that you can turn stuff in on schedule.)

Most certainly. Once you have a book or two to show, things tend to become a lot easier since the editor can see that you don't just have the necessary skill, but are also capable of meeting a deadline.

I assume conventions like this one or the book fair in Frankfurt are excellent places to make contact. The panelists talked a bit about how they got tie-in jobs... approach the publisher at conventions, try to get the name and contact info of the person responsible for the tie-in titles if she's not there, send some samples. Or just call and ask for the contact info and whether they accept unsolicited work samples.

I'd avoid the word fan fic (though seriously, so far my own publishers here have had not the slightest problem with the fact that I do fan comics on the side... but from what I gather it could different over there.)