Furthermore, I imagine that this can backfire reaaaly hard: if you manage to develop a strong revulsion for unproductive activities but still can't force yourself to stop browsing reddit (or whatever your vice) then you run a big risk of hitting a willpower-draining death spiral.

That's basically what happened to me: I taught myself to feel guilty whenever I was relaxing and not working, but just the fact that I was feeling guilty about not-working didn't make me any more motivated to actually work. So I would repeatedly get into situations where absolutely nothing felt like worth doing, so I accomplished basically nothing and felt miserable for the whole day. Cue an extended burnout that took me several years to properly recover from.

Oddly, it feels like one key part of my recovery has been to train myself to feel as unguilty as possible about any recreational activity. That way, if I really need a break I can take one, but if I'm on a break I can also honestly ask myself whether my break has gone on long enough and whether I'd want to resume doing something more productive now. Though I'm sure if that's quite right either - it's more like I'm more able to trust that my motivation to do something relaxing will naturally fade after a while, to be replaced with a motivation to be productive again, without me necessarily even needing to watch myself. And of course, since I don't need to actively watch myself, the relaxation may happen faster since I can focus on it more fully. (Of course, sometimes it does take longer, and the key is to be completely fine with that possibility, too.)

The main mechanism here seems to be that guilt not only blocks the relaxation, it also creates negative associations around the productive things - the productivity becomes that nasty uncomfortable reason why you don't get to do fun things, and you flinch away from even thinking about the productive tasks, since thinking about them makes you feel more guilty about not already doing them. Which in turn blocks you from developing a natural motivation to do them.

So if someone did go by this mindhacking route, they should be very careful to avoid developing guilt. The guest who had developed a dislike for fritos didn't dislike them because eating them made her feel guilty: she disliked them because she had started noticing features in them that she felt were repulsive. Also, I suspect that "actively pay attention to the features in productive tasks that are desirable" is just as important an component as noticing the displeasing things in non-productive tasks. If we assume the opportunity cost model of willpower, then your motivation to do something is proportional to the difference in estimated value between that thing and the second most highly ranked thing, implying that increasing the perceived value of the productive things can be even more efficient than decreasing the value of other things. (Guilt in this model would act as a negative modifier to the values.)

Also closely related posts: Pain and gain motivation, It's okay to be (at least a little) irrational.