I have seen the Merbeck in square notes in reproduction in very early BCPs. You might try that route. I, too would like to have this. The notes are square, as any liturgical chant would be at that date, The notation is square, as any liturgical text at that time would be, but one wouldn't call it exactly 'Gregorian'.

By the way, if someone has this Mass in modern notation, I am still interested.

In modern notation it's in the back of the 1940 and the front of the 1982.
If I still had multiple copies of Canon Douglas's St Dunsan's Kyriale I would send you one or two - but I gave a lot of extra stuff to others in the last couple of years. You could probably get one from Amazon.

I just checked Amazon and they have several starting at $59.

I doubt that Willan's Mass would be available in neums, since it's a modern, metrical composition: Maybe someone has transcribed it into 16th century mensural notation? It, along with the rest of Willan's music, is copyright, so I very much doubt it.

The Merbecke Communion Service should be readily available online, in modern or square notation. I put together a version of Merbecke's Gloria for the 2011 translation (attached), which might be of interest.