Perpendicularity callout with curved datum

[bc23]

I re-created this drawing for illustration purposes. All the flat sides/surfaces are actually not flat (so cannot be used as datums).
The intention is for dia4 hole to be perpendicular to curved surface (R10). But is this FCF correct? (whether by ASME or ISO)
If Datum A was re-defined as a specific line on this curved surface, would it be legal? (But it would be difficult to measure also)

 

[3DDave]

How odd.

3.46 PERPENDICULARITY
perpendicularity: the condition of a line element, surface, tangent plane, feature’s center plane, or feature’s axis at an implied or specified basic 90° perpendicular) angle relative to one or more datum planes or datum axes.

So, no to specific line.

However, if there is perpendicularity to an axis then a plane can be constructed through the axis and where that plane intersects the outer surface, one can create a tangent plane. However, that feature might be anywhere on that plane and need not intersect the axis. If that is acceptable, that feature control frame will do it.

A position geometric characteristic tolerance would act similarly, but restrict the true position to passing through the axis.