I thought it OK, but a bit basic and searched some more. I couldn't find anything else.
So I gave it some thought and came up with this.
The melody of the grace itself (Benedic Domine etc) is taken from the first line of the chant for the Ubi Caritas. I chose that for a number of reasons. One is that I love the tune; a second that it is a chant for Maundy Thursday, and I liked the link between grace and the Last Supper; and the third is that it quickly became apparent that the structure of the melody would need little adaptation to fit the words - apart from the Signum Crucis (In nomine Patris) and the concluding Per Christum Dominum Nostrum.
For these, I turned to the Gloria Patri for the Introit of the Mass, in Mode 6 (as the Ubi Caritas melody is in mode 6) and adapted them.
I think the whole thing works ok: not super smooth, but singable and recognisably Gregorian in feel.
If anyone can do better, by all means do!
Here is the Grace and chant written out. If you would like a .pdf of it, simply email me (Benny.trovato [at] gmail.com) or leave your email address in the Comms Box.