I think it might be helpful if Glyphs could ignore the headline of Devanagari and similar scripts for the purpose of displaying metrics values.
For example, if the headline of a font fills a space between 600 and 680, that value could be defined in a font similar to the way alignment zones are defined. Then the sidebearing values stop being L:0 R: -10 and display the actual sidebearing value relative to the body of the glyph. This might also be useful with styles of Arabic that have a straight horizontal baseline.
Could be done with a plugin. Or two actually: one for displaying the headline, the other for inserting at export time. Give me a few days.
And what is the smartest way to determine which glyphs get a headline and which do not? Include clause in a parameter? Or informed guess based on the glyph name?