Challoner was consecrated a Coadjutor Bishop in 1741; succeeded to the London Vicariate in 1758; and died in 1781. The most interesting of his memorabilia carefully preserved and beautifully exhibited at Ware, is a small sheaf of handwritten papers, labelled 'Challoner's Lectures at Douay' vel sim.. I was only able to see the top sheet (the case was of course locked), but these were clearly not 'lectures'; I think some well-meaning but unlatinate Keeper must have been misled by the word 'lectio'. What they are is something which seems to me potentially much more interesting: a Supplementum pro Anglia of a Breviary. Readers will know that clergy reciting their Office follow the Calendar of the Universal Church except when the local supplement provides for the observance of local Saints or Devotions. Nowadays, each Diocese has its own Supplementum, which is bound in at the back of each volume of the Breviary. But, in a Breviary I possess of 1874 (formerly in the library of my late dear friend Fr Michael Melrose, Vicar of S Giles, Reading, who was denied entry to the Ordinariate by his sudden death), the Supplementum is not diocesan but national (Anglia). I wonder when this Supplementum was created, and by whom. Does this evidence from Ware give us the answer: By Challoner?
You will recall that these papers of Challoner's are handwritten. It is not easy to know why a Vicar Apostolic was using handwritten propers to say his Office if a printed Supplementum, approved as such things were by the Sacred Congregation of Rites, had been available. And here's something else. These sheets contain, I think in Challoner's hand, alterations, emendations.
My theory, quite possibly wrong since I only had ten minutes and only saw the top sheet and have been able to conduct no relevant researches, is that Challoner may here be constructing an English National Supplementum himself, and then tweaking it year by year as he himself prays his own drafts. I would be fascinated if those more erudite in such sidewaters of the history of the Roman Rite could explode or supplement my thoughts with data (such as, of course, a printed Supplementum pro Anglia predating Challoner!). And when did the SCR start authorising local Supplementa?
If, however my preliminary musings are right, then what we have here is a Bishop, the presiding Liturgist of his Particular Church, still exercising a right of liturgical composition (not so terribly surprising in a century in which the 'Gallican' bishops of France continued to do exactly that; cfr. ex. gr. the Missale Parisiense of 1738). And we would also have one of the last examples of the transmission of liturgical texts manu scripti. And we would have a creative intervention of Challoner's not unlike his own radical revision of the Douay-Rheims Bible; his composition of The Garden of the Soul, Brittania Sancta, the British Martyrology; and so many other works.
He was providing a complete Catholic culture for an English Church which did not yet possess one.