[soapcake]

A few studies on HQ and stability for those still interested.

https://www.researchgate.net/publication/282053731_Compatibility_study_between_hydroquinone_and_the_excipients_used_in_semi-solid_pharmaceutical_forms_by_thermal_and_non-thermal_techniques 

A study on formulations, particle size, and stability. A pharmaceutical base, called Nourivan which is used to compound HQ and other actives prone to oxidation, along with Dimethyl Isosorbide was the most effective. 

https://www.mdpi.com/1999-4923/11/4/167

The ingredient list for Nourivan: Purified water, cetearyl alcohol, polysorbate 60, C13-C16 isoparaffin, C12-C14 isoparaffin, C13-C15 alkane, glyceryl stearate, polyacrylate 13, polyisobutene, polysorbate 20, polyurethane-39, stearyl behenate, cetyl alcohol, ascorbic acid, 0.1% benzoic acid, 0.1% sodium bisulfite, 0.1% sorbic acid, 0.1% tocopheryl acetate. pH 3.5

[soapcake]

A quat would be nice. Also marshmallow root gives a very lovely slip to shampoo, although I have only tried this in a solid shampoo. I found a very small amount brings a nice conditioning feel.