Sorry, it's not possible to associate more than a material to each shape of a 3D model. But MTL, DAE and 3DS file formats allow to cite materials which are not actually used, so maybe using some special naming conventions on materials (like platform, platform_sweethome3d_alternative_platform2, platform_sweethome3d_alternative_platform3,...), the program could store these materials to show them in a new dropped down list in the Material choice dialog box.
In your case, this would imply to edit the MTL or DAE file accordingly (editing a 3DS file would be more difficult since it's a binary file), and pay attention not to lose these changes when you export again your 3D model after an update in the modeler program.
It could be also an interesting option for furniture makers if they want to limit the choice of materials on a piece of furniture, for example to let users choose only white, black or blue for a part of this piece.
Another solution I've been thinking for a while, would be to make your solution 3 more user friendly: instead of displaying the 8 numbers which gives an ugly model at screen in the hope that the user will make 7 of these materials invisible, Sweet Home 3D could allow to make visible some invisible materials (defined for example with d 0 in their MTL file). Therefore, your platform would look nicer when loaded, but the user will have to guess that there could be some invisible shapes that he could use instead of the default number.
I wanted to program this feature to propose a choice of different handles on kitchen cabinets showing a default handle when loaded. The problem of this feature is that it's quite advanced for the end user who has to explore the list in Material modification dialog box and guess that there are some invisible materials that could be made visible instead of other materials.
Hope that advanced users here understand all this
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Emmanuel Puybaret, Sweet Home 3D creator
