Then again freelancers are usually expected to already know the stuff they are supposed to work on. So it would tend to be "not challenging" by definition :-(
By the way, I disagree on freelancing not being challenging because you should already know what you are doing. You are always having to learn new things and do things that you aren't expert at. Maybe you need to bust out something you don't have a lot of experience with because there is nobody else on your team to do it. Sure, you could outsource it, but often enough you find you will be tackling that thing yourself.
More importantly, freelance jobs are challenging not because of the technical stuff, but because of dealing with people and running a business. That never gets easier. You get more experienced with it, but it's always a challenge. If you don't feel challenged, then you probably need to up your game.
TheLadders.com has made a nice job of segmenting off and servicing a "100k" jobs market, but salary is one thing that's pretty easy to determine in a job. It may be a horrible job, but if the ad says "$130k", you know what you'll get paid.
"bad ass" is in the eye of the beholder too - your job is rotting your brain. Someone else coming in to the field might consider it a real challenge and growth opp. I don't know your job, so perhaps that's not accurate, but I've seen it myself. Other people have come in to positions I've hated and they've thrived.
On the whole it sounds like a tempting idea, but my thought is that, without some serious time/effort to filter and interview the employers, it'd be really hard to do well.
Now... you could charge the employer a decent fee, but give them a few video slots, with interviews/pitches from various team members you'd be working with - turn the interview process on its head slightly. Companies willing to go through the process of really selling themselves and a particular position beyond just one simple list of bland bullet points might be considered badass just on that point alone.
[email protected] if you'd like to consider dicussing or pursuing this more.
Rereading your post, I don't think the issue is particular jobs - I think it's you. You're ready for freelancing. The most challenging/particular things (I've found) have come from short term consulting projects. Come in , problem solve, execute, leave before rot sets in. This is the sort of thing we'll be promoting at http://indieconf.com in November :)