SOLIDWORKS 2019 can now link custom properties of an opposite-hand mirrored assembly. Mirrored part files already had the ability to transfer the custom properties. Now custom (or configuration-specific) properties can be transferred to the new assembly file.
The Transfer options did show the option for Custom Properties, but this was only for the part files. Now the assembly files themselves will have linked properties to the parent assembly file. Each property can be unlinked if overrides are required.