This seems plausible to me. Note that I consider trying to do things like proving theorems to be "novel situations", whereas simply learning what the theorem says doesn't feel like it would be as effective at increasing fluid intelligence, if at all. Since CS (I assume you mean programming here, since I consider theoretical CS part of mathematics) often requires you to put your thinking cap on, so to speak, I can very easily see that buffing fluid intelligence as well. Which, of course, requires that fluid intelligence increase be possible in the first place. As you said, it certainly feels like doing math/CS buffs fluid intelligence, but...
According to my notes from the above Doidge book, things we can change include "memory", "processing speed", "intelligence", and "ability to learn."
While I'm uncertain whether the rest are well-accepted, being able to improve memory is well-accepted among cognitive psychologists. They trained this one subject to a digit span of 80. Also, one feature of mnemonic techniques is "speed up", or the ability to improve with practice, hence memory competitions. Method of loci is just ridiculously powerful (for the narrow and typically unhelpful types of information that can be method-of-loci'd).
Your job, should you choose to accept it, is to comment on this thread explaining the most awesome thing you've done this month. You may be as blatantly proud of yourself as you feel. You may unabashedly consider yourself the coolest freaking person ever because of that awesome thing you're dying to tell everyone about. This is the place to do just that.
Remember, however, that this isn't any kind of progress thread. Nor is it any kind of proposal thread. This thread is solely for people to talk about the awesome things they have done. Not "will do". Not "are working on". Have already done. This is to cultivate an environment of object level productivity rather than meta-productivity methods.
So, what's the coolest thing you've done this month?