"Is the New World Translation the only Bible to phrase John 1:1c as "the Word was *a* God"?"
Consider the following:
1808: “and the word was a god.” - The New Testament in an Improved Version, Upon the Basis of Archbishop Newcome’s New Translation: With a Corrected Text.
1864: “and a god was the word.” - The Emphatic Diaglott, interlinear reading, by Benjamin Wilson.
1928: “and the Word was a divine being.” - La Bible du Centenaire, L’Evangile selon Jean, by Maurice Goguel.
1935: “and the Word was divine.” - The Bible—An American Translation, by J. M. P. Smith and E. J. Goodspeed.
1946: “and of a divine kind was the Word.” - Das Neue Testament, by Ludwig Thimme.
1958: “and the Word was a God.” - The New Testament, by James L. Tomanek.
1975: “and a god (or, of a divine kind) was the Word.” - Das Evangelium nach Johannes, by Siegfried Schulz.
1978: “and godlike kind was the Logos.” - Das Evangelium nach Johannes, by Johannes Schneider.
Even Origen, the most knowledgeable of the early Christian Greek-speaking scholars, tells us that John 1:1c actually means "the Word [logos] was a god". - "Origen's Commentary on John," Book I, ch. 42 - Bk II, ch.3.
Jehovah's Witnesses have been criticized for allowing the indefinite article (a) at John 1:1c. However, the true fault lies with their critics. It is the other way around...the absence of the indefinite article at John 1:1c has been purposely mistranslated in most Trinitarian-produced Bibles to fit their doctrine that Jesus is God.