I used to have this trick where by I use the default textures. When modeling in Sketchup I would use the default texture (DDS converted and flipped in to Jpg). When I load in the Collada in to ModelconverterX, I used to change out the jpg for the DDS in the material editor. After that I would select the Bump, Nightmap and Specular.
Ok, right now you model is linked to the .jpg texture and will remain that way until you recompile it. You can reassign it in the Material Editor, but it won't show unless MCX can easily find this new texture and it isn't enough that you include the path or point to it - that information will only be used if you actually compile the model. To see your new texture when you select it in the Material Editor/Properties, it must reside in the same folder as the model or directly adjacent, as in parent folder> model/texture folder.
I believe my procedure to be not only simpler, but also much more effective. Early during modelling in SU, I send my .jpg to Photoshop to edit. When I edit the image, I use multiple layers for shadows, wear, in fact many of my texture edits are simply a new layer superimposed over the unedited part of the texture. As an example with your building model, I can move the windows to exactly line up with porticoes on the model. After editing, I save the .psd texture to my SU working folder, then I go into SU Material window, select new file for my working texture and import the layered .psd into my SU work space. I can have the entire building, including zoomed in close ups of the mailboxes and transparent windows, mapped to the same master texture. Obviously I use this technique to also create the night texture and save as the same file with the "_LM" suffix. Upon export, I get a Collada mapped to Photoshop layered textures. I pick this up with MCX, where I can further refine and tune my textures because MCX also supports Photoshop layered image files. When I have established my material properties in the Material Editor, I switch to the Textures tab and save all the working texture(s) to .dds format and finally export a .mdl with .dds textures.
Basically, I am suggesting that switching to .dds with the Material Editor Properties tab is tedious when you can simply save to .dds from the Textures tab; granted for either procedure to stick, the model must be compiled to that state..
