Besides in the sense of fun or amusement, the word humour may also carry the meaning of a state of feeling, a ‘mood’, like in ” You seem to be in good humour today”. That does not mean, or even imply any notion of, amusement or merriment.
When in a good mood, there is an agreeable (and so a non-conflicting) attitude. In the instance that you quote, ”learning how to maintain good humour around the practice.” should be taken in this sense: with calm and composure, realistically, with equanimity. If there is hunger and tiredness today and no practice is possible, the right conditions will be there soon, perhaps even tomorrow. Where is the fun, the ‘humour’ here? There are days at a stretch when I cannot apply myself either to reading Dhamma or participating at the Forum, though maintaining mindfulness all the while. The energy is just not there.
The sutta you are quoting Thanissaro on is Kusīta-Ārabbhavatthu Sutta (AN 8:95). It is so, is it not? However, Suttacentral lists this as AN.80: Grounds for Laziness and arousing Energy. I cannot see how the Bhikkhu can read humour in the conventional sense here at all.
What the Buddha is saying is that all those excuses (though REAL) should be overcome by constantly reminding oneself of the Goal . “They rouse energy for attaining the Unattained, achieving the Unachieved, and realizing the Unrealized.” (Capitals mine). Again, no hint of fun or hilarity here at all. Who knows whether any bhikkhu, or lay person, you or me or anyone else will still be alive in 1 hour’s time, in 5 seconds time? It is not a humorous situation at all.
However, there is a sutta where the Buddha actually smiles. It is when He saw some cows in the distance. I have been unable to trace the sutta for reference, so if you come across it you may read it for yourself. It will be worth it in distinguishing between the ways of a Buddha and those of ordinary humans.
A bhikkhu asked the Buddha: ” Lord, why have you smiled”. Now an ordinary person would smile on seeing cows either condescendingly, looking down on them, thinking: ‘Oh, poor cows. But I am human’ ,or, which is better, fondly: “What peaceful animals”. Or, if it is a cowman, their owner “Their milk is mine, their meat is mine, I own their lives” .
But the Buddha replies that each of those cows had been a King of the Devas. Now, if one pauses at this sentence before reading the next, one may think that the Buddha was showing compassion (Karuna) for the cows. But then He concludes that none of the bhikkhus there would ever be born a cow again (meaning they all had attained at least the Sotapanna Stage). So in fact, what that smile ‘gave away’ was the sympathetic joy, Mudita, for the bhikkhus.
With a Buddha even so much as a smile has great significance. Every act by body, word or mind is out of compassion. Teaching the Dhamma is no lighthearted matter. Much, much less a humorous one.
May the Blessings of the Buddha, the Dhamma and the Sangha guide you to final Liberation.