It seems to me you would like to put constraints on the position of the center C of your largest empty circle (LEC).
The abstract of: TOUSSAINT, Godfried T. Computing largest empty circles with location constraints. International journal of computer & information sciences, 1983, 12.5: 347-358, says:
In particular, it is shown that if the center of C is constrained to lie in an arbitrary convex n-gon, an O(n log n) algorithm can still be obtained.
Also: CHEW, L. Paul; DRYSDALE, Scot. Finding largest empty circles with location constraints. 1986. says:
We also improve on an algorithm given by Toussaint for computing the LEC when the center is constrained to lie within an arbitrary simple polygon.