dropAnchor on caps-height or x-height?

[vinuel]

dropAnchor has some really helpful alignment expressions! I wonder if it is possible to place anchors at the cap height or x-height.

'Top' places the anchor at the highest point of a glyph, this results in different anchor positions for glyphs with overshoot, but anchors should all be at the same height in a font and it would be great to do so by an expression.