In my view, it is a power of the GM. To further quote from Br. Mackey:
'an Occasional Lodge,' specially convened by him, and consisting of such Master Masons as he may call together for that purpose only; the Lodge ceasing to exist as soon as the initiation, passing, or raising has been accomplished, and the Brethren have been dismissed by the Grand Master.
"In 1731, Lord Lovell, being Grand Master, he 'formed an Occasional Lodge at Houghton Hall, Sir Robert Walpole's House in Norfolk,' and there made the Duke of Lorraine, afterwards Emperor of Germany, and the Duke of Newcastle, Master Masons.
"The initiation, passing and raising of Frederick, Prince of Wales, in 1737, was done in an 'Occasional Lodge,' over which Dr. Desaguliers presided, but this cannot properly be called a 'making at sight,' because Dr. Desaguilers at the time was a Past Grand Master, and not the actual Grand Master at the time. He most probably acted under the dispensation of the Grand Master, who at that time was the Earl of Darnley.
"In 1766, Lord Blaney, who was then Grand Master, convened an 'Occasional Lodge,' and initiated, passed and raised the Duke of Gloucester.
"Again in 1767, John Salter, the Deputy then acting as Grand Master, convened an 'Occasional Lodge,' and conferred the three degrees on the Duke of Cumberland.
"In 1787 the Prince of Wales was made a Mason 'at an Occasional Lodge, convened,' says Preston, 'for the purpose at the Star and Garter, at Pall Mall, over which the Duke of Cumberland (Grand Master) presided in person.'
"It has been said, however, by those who deny the existence of this prerogative, that these 'Occasional Lodges' were only Special Communications of the Grand Lodge, and the 'makings' are thus supposed to have taken place under the authority of that body, and not of the Grand Master. The facts, however, do not sustain this position. Throughout the Book of Constitutions, other Communications, whether Stated or Special, are distinctly recorded as Communications of the Grand Lodge; while these 'Occasional Lodges' appear only to have been convened by the Grand Master for the purpose of making Masons. Besides, in many instances, the Lodge was held at a different place from that of the Grand Lodge, and the officers were not, with the exception of the Grand Master, the officers of the Grand Lodge. Thus the 'Occasional Lodge' which initiated the Duke of Lorraine was held at the residence of Sir Robert Walpole, in Norfolk, while the Grand Lodge always met in London. In 1766 the Grand Lodge held its Communication at the Crown and Anchor, but the 'Occasional Lodge' which in the same year conferred the degrees on the Duke of Gloucester, was convened at the Horn tavern. In the following year, the Lodge which initiated the Duke of Cumberland was convened at the 'Thatched House' tavern, the Grand Lodge continuing to meet at the Crown and Anchor.
"But, without doubt, a conclusive argument may be drawn from the dispensing powers of the Grand Master, which has never been denied. No one has doubted, or can doubt, the inherent right of the Grand Master to constitute Lodges by Dispensation, and in these Lodges so constituted, Masons may be legally entered, passed and raised. This is done every day. A constitutional number of Master Masons applying to the Grand Master, he grants them a Dispensation, under authority of which they proceed to open and hold a Lodge, and to make Masons. This Lodge is, however, admitted to be the mere creature of the Grand Master, for it is in his power at any time to revoke the Dispensation he had granted, and thus to dissolve the Lodge."
End quote.