The Polytonic Greek (SIL) keyboard on iOS shows the Pamphylian digamma Ͷ ͷ, in the position of v on a standard QWERTY keyboard. This variety of digamma is rarely ever used.
Please consider replacing it with the far more common digamma Ϝ ϝ, used in some dialects as well as many pedagogical materials and linguistic work. It also has a numerical value of 6 in Greek numerals.
If back compatibility is a concern, maybe you can use a long press menu to accommodate ͷ/ϝ.
Relevant unicode: U+03DC Ϝ, U+03DD ϝ
(Also, there’s a bug in version 1.8.1 of the keyboard where the shifted space key produces ς, not a space)