I am not convinced we need the longhands, but we should certainly have a shorthand. The simplest grammar would probably be this:

text-edge: leading | normal | <over-metric> <under-metric>
<over-metric>: text | cap | ex | ideographic | ideographic-ink
<under-metric>: text | alphabetic | ideographic | ideographic-ink

I suppose we could also make the second value optional, and map it to the same value as the first one in the cases where the pairs exist (text, ideographic, and ideographic-ink). What the implied second value would be if you picked cap or ex as the over side is less obvious though. Maybe cap, which is a broad metric, might be matched with text for the under side, while ex, which is a tight metric, would be with alphabetic.

I've drafted it up with longhands for now; defaulting missing keywords to text because that's the safest.