Would the receipt of Units of Beneficial Interest by the settlor (or whoever transferred title of an asset to the trust) be considered income for tax purposes?
No, because the units are not traded on any public exchanges, and thus have no publicly determinable monetary value. The value is given to them by the officers of the trust, within the boundary of the private trust contract. It can therefore not be questioned or interfered with by any outside party. It is private and confidential information. When the value can thus not be determined by an outside party, there is no way for that outside party to make an intelligent claim to it.
And finally, the very fact of having received the units in the first place is private. There is no need for the settlor or whoever transferred assets to the trust to disclose the receipt of the Units of Beneficial Interest to any public agency or any outside party. It is private knowledge protected within the trust contract relationship. This bestows an even more significant protection to the transferor. If no one knows that someone has received Units of Beneficial Interest, how can anyone even contemplate claiming them?