I don't think requestScan is relevant, in 8.1 icons/themes/fonts are handled by bespoke-to-Perspective "asset" managers that do their own file watching. In 8.3 this content is moved into the general purpose config system, so it still won't be system.project.requestScan but there will (eventually) be an equivalent system.config function.
As far as I know the icon picker preview is only generated once on designer startup. (My past self agrees, and actually did the research: Custom Icon Library - #5 by PGriffith)