You're right that the logistics are indeed more complicated in a polyamorous relationship; that's probably one of the hardest parts of polyamory. But I'm not sure I agree with:

There's no feasible method for someone to commit to a set distribution of time/effort/attention between partners. I'm not saying there should be, just pointing out that such things can't realistically be budgeted for or enforced.

Even in monogamous relationships there are time and energy conflicts. People need to schedule their time between their partner, friends, family, work, hobbies, and personal time. The only method I know for committing to and scheduling time is to make a schedule with your partner(s) and discuss it with them regularly to make sure you're keeping to it. You can schedule slots of time, and then if you're missing that time with them, there's a problem in that relationship and it needs to be reconsidered.