I would like to zero-pad a time duration to be HH:MM:SS but it appears this is not possible. A stored cell value such as "0.019502314814814816" will only zero-pad to H:MM:SS when Format > Number > Duration is applied to that cell.

Custom number formatting would presumably give the flexibility to fix this (and is a basic feature already present in competitor sheet editor software).

Support for custom number formats in Sheets would vastly improve its usability. I have seen several requests for specific currencies among the feature requests here, and a custom format option would address all of those, and more (like adding a unit to a number, for example kg, mL, km/h, etc.). Google Sheets has Custom currency, Custom date and time, and Custom number format, where you can define a format with placeholders and apply it to a cell.

Seriously? first i post 2 suggestions together and it gets declined for being multiple suggestions, and now it gets merged together after i post them seperately -_-
and they aren't even in the same category.... they are two completely different suggestions