I've been giving a lot of thought to this idea.
What's the worst that can happen? I mean, when you are 'felting' you want the fabric to shrink, and the fibres to matt together. So what harm could the tumble dryer possibly cause to that process?
The only concern I really had was that it might somehow crumple the finished fabric. But I have an iron (don't tell Mr Goldfish, or he'll expect me to start using it on a regular basis) so I could smooth out any crumples.
Anyway. I decided to experiment.
I let one jumper sleeve air dry... tumble dried the other and compared the finished appearance. The sleeve which went through the dryer shrank a teensy bit more than the air dried one. Only the smallest amount. Like 1cm difference over the whole length of the arm. This difference will vary with each individual jumper, but it's not enough to worry about.
So the next jumper I felt will be dried in the tumble dryer. That way I can avoid the smell of wet wool in my house for days on end. Especially over winter. I'll keep you updated on my success (or lack thereof)
So here is a close up of the jumper before felting. It's the reverse side, and you can clearly see the ridges formed by each stitch.
And afterwards, it look like this... you can only just make out those ridges.
Now to decide what to make with it...