Just for the sake of completeness: after a few more trials I found that although DXT1 textures can be used (after applying textures in PNG format and then replacing by DDS - DXT1), they definitely do not show on the 2D/3D views in AC3D. The Xplane tool DDSTool.exe converts the PNG textures into DXT5 format, which are properly shown in AC3D.
Unfortunately DXT5 textures are - depending on their graphical content - much larger than DXT1, which is not very practical for obvious reasons.
As a suggestion to the devs: It would therefore be great if AC3D could also show textures in DXT1 format
