Since spectral thread is the only perk that affects the more/less modifier for max starlight charge, having enough perk effectiveness takes max starlight charge to below 0 and of perks that increase it will multiply the negative. This can be done with this perk loadout and at least 2 armor pieces with perk effectiveness from a Horologium engraved glass. (stats page with two +20% perk effectiveness armor pieces) When starlight charge is negative, attempting to use items messes with the cost preview and as expected, items and mantles that require starlight charge do not function.

I imagine a hard minimum of zero could be set to starlight charge, but also maybe more things could be added that influence more/less max starlight charge.

as of AS version 1.12.6 on forge version 31.2.9