We say that when the Buddha was a bodhisattva, he vowed to go to Naraka to save beings suffering there. Compassion is the wish that others be free from suffering. Great compassion is the wish that all sentient beings be free from suffering. Superior intention is taking upon oneself to [do what it takes] free others from suffering and establish them in happiness. So, strictly speaking, here it is a case of 'superior intention'.
In the Tibetan Gelug tradition, we recite the Lamrim prayer of the Lama Chöpa every two weeks. At some point, it reads:
Even if I must remain for an ocean of eons in the fiery hells of Avici
for the sake of even just one sentient being, I seek your blessings to
complete the perfection of joyous effort to strive with compassion for
supreme enlightenment and not be discouraged.
Still, as you say
- One harvest what one has sown / None can take someone else's karma upon him
- Buddha can not give enlightenment to others.
- Dharma is not heard in Avici. But there is a story of a hell being generating compassion for another hell being's suffering. According to that story, the other hell being was an emanation of a buddha, and he meant for the first to generate compassion so as to create a cause of happiness.
This does not prevent us from aspiring to take upon ourselves the suffering of others, and to give away our happiness and its causes (merits). And this is the whole point: we cultivate an intention, because this intention will be a cause of achieving enlightenment.
It is just like the perfection of generosity: the virtuous intention to give. One (arya bodhisattva) can cultivate the perfection of generosity, that is the intention, even when there are still needy people in the world. Cultivating generosity does not mean eradicating poverty. Similarly, taking it upon oneself to establish sentient beings in enlightenment does not mean establishing them in enlightenment. The best a buddha can do is turning the wheel of Dharma, out of his omniscient mind, and that turning will be but a condition (not the substantial cause) of others' enlightenment.