There is a parameter: --user-data-path
However, this only works if you have a shortcut that points to the actual EXE of OpenRCT2. Modifying the Launcher shortcut will not have an effect.
If you just want to use the Launcher, the symbolic link is probably a better idea.
You can already create custom JSON objects, but they can't be embedded in the SV6. That problem will be solved when we have our own save format.
Support for PNG images was added this last month. You can now load images from CSG1, G1, an object DAT and embedded PNG. The default objects will continue using DAT images.