[Formendacil]

When Gandalf says "an Elvish script," he means just that--an Elvish script, i.e. writing system.

As near as I have ever been able to tell from the Appendices, Elvish letters (script) was the main writing system of Middle-earth in the Third Age, and its use was more-or-less parallel to the use of Westron as the common tongue. And, if I have had the right impression all these years, the Book of Mazabul was in the Common Tongue*.

Thus, it follows, the various Dwarves writing in it might have used either the Runes, which were especially familiar to them, or the Elvish letters, which were the common way of writing that language.


*It really HAS to be the Common Tongue that is in the Book of Mazabul, for a couple reasons: 1. Gandalf is not fluent in Khuzdul, as evidenced by his remarks outside the Doors of Moria that he would have to call on Gimli if he were to need words in that tongue, and 2. Khuzdul was not, especially in these later days, apparently something that the Dwarves shared much--if at all--with outsiders. Granted, the logbook of the expedition was hardly intended to be sent to George, Allen, & Unwin for mass printing, but it does seem to have been intended as a record that COULD be shared with anyone who might come, either to find the Kingdom in splendour, or as a record of their failure as the case ultimately ended.