I have been trying to figure out how the old renders might be accessed for download when needed, so here's my list of potential workarounds. Will any of these work (without damaging the store)?
1. When wanting to download a grey sale render, toggle the store setting from using material renders to only using the grey renders. Download the grey render you want, and then change the setting back to use material renders. Possible danger: all renders and previously uploaded images get corrupted (captions get lost and other bad things happen).
2. Is there a naming convention for all the renders that are generated such that you could take the web link address for another material render and perhaps just change a few numbers in the link address to access the grey render? In other words is there a material code embedded in each render link that can be changed to get a different render? This seems like a great immediate solution to get the grey renders without worrying about the rest of the store. No additional work or worry for Shapeways and people who need the old renders can easily get to them.
3. Generate the grey render the same way Shapeways does, perhaps using a script in a 3rd party program that will render things consistently with the same appearance in a grey scale mode. Ick, way more work than I want to do.
4. Open a bogus store just to generate grey scale renders. Set up the store to not use the material renders, upload a model to it when you want the grey scale image, download the grey scale image, and then delete the model from the bogus store. Also no additional work for Shapeways but not as simple as solution #2.
Any other ideas?