Justicar has been used several times before, and in D&D too (at least one or two prestige classes, and earlier IIRC). Though the proper form is probably Justiciar for English, it's still been spelled as Justicar in various places. Also,
according to Wikipedia: "Its chief executive was variously known as the Justicar, the Lord Deputy and, from the seventeenth century, the Lord Lieutenant of Ireland (also known as the Viceroy), who was both the English (pre-1707)/British government representative in Ireland and the personal representative in Ireland of the King." from the 'Dublin Castle administration in Ireland' article.
I don't particularly care which is the most accurate English spelling, since both versions have been used enough for me to ignore as just something odd like the way British English spells color and armor as colour and armour, while American English says "screw those extra letters, we know how it's pronounced."
I always pronounce it as justisar anyway, treating the C like an S, because that's just how it seems like it'd be pronounced in English. Treating it like a K or a KI seems clunkier in speech.