Historically there was a type of canon known as a prebendary canon.
Kinds of canons
Canons are divided in the following manner: (1) Cathedral canons, who, attached to the cathedral church, form the senate or council of the bishop; collegiate canons, who perform the canonical office in the church to which they are attached, but are not connected by reason of their office with the government of the diocese. (2) Prebendary canons, who have a prebend or fixed income attached to the canonry; simple canons, who have no prebend. (3) Canons de numero, i.e. those of a church the number of whose canons can neither be diminished nor increased; (4) supernumerary canons, who are assistants to the canons de numero. The supernumerary canons are subdivided into three classes, viz. (a) those whom the Holy Father appoints and who will receive the first vacant prebend (expectant canons); (b) honorary canons (for these see the Constitution of Leo XIII "Illud est proprium", 21 Jan., 1894, and the recent decree of the Congr. of Rites, 14 Nov., 1902), and (c) canons who are added on the founding of a new prebend. Formerly the chief distinction was that made between the secular and regular canons. Regular canons, as forming the council of the bishop, are now almost obsolete, and the special regulations by which they are bound, their rights, privileges, and duties, are treated fully in works on canon law. The special status of canons in English-speaking countries will be considered later.
Wikipedia has the following to say about what a prebendary canon is:
A prebendary is a member of the Anglican or Roman Catholic clergy, a form of canon with a role in the administration of a cathedral or collegiate church. When attending services, prebendaries sit in particular seats, usually at the back of the choir stalls, known as prebendal stalls.
A prebend is the form of benefice held by a prebendary: historically, the stipend attached to it was usually drawn from specific sources in the income of a cathedral's estates. In the 21st century, many remaining prebendaries hold an honorary position which does not carry an income with it. - Prebendary
Not finding a proper definition of demi-prebendary canon, I would believe that they are simply honorary prebendary canon, who do not get an income from this honorary position.
Similarly, a claustral prior is a demi-prelate. Under certain circumstances, he may perform some of the duties of an abbot, following the absence or death of his superior, yet is not an abbot. Thus his actions are very limited.