It will be worth it to give more thought to this.
When brahmas, devas, and even yakkhas (‘nature spirits’, we may say) communicate with humans, they assume a human or other form, TOGETHER WITH THE FACULTIES OF THAT FORM.
Taking first the case of Brahma Sahampati, he could ‘listen’ to, he was able to sense what the Buddha was thinking, His doubt about whether to proclaim the Dhamma or not. If it were just a matter of a telepathic exchange, he would just have responded in like fashion FROM that brahma realm to made his noble entreaty to the Buddha to go ahead and preach the Dhamma. But no; brahma Sahampati disappears from that brahma realm and reappears in front of the Buddha. He assumes a human form: he ‘arranged his robe over one shoulder, knelt with his right knee on the ground, raised his joined palms toward the Buddha, and SAID…”
So it is not that communication with, in our case, the Buddha takes place with the manomayakaya alone. That would have been the obvious response.
The same can be said in the case of Ugga here. As long as there is a manomayakaya, I see no reason why all orders* of brahmas should be unable to assume any form they wish. Yakkhas can do it.
But we find many accounts of humans with magga phala who ‘saw through’ the assumed human forms yakkhas and devas put on. Any other would have been easily taken in by those assumed forms.
* afterthought: except those in the asanna realm, as far as I can make out.