I'm not a designer, so please enlighten me. What kind of data can be held in a single image for a PSD to be 900 megabytes? Is it a single PSD for hundreds of web pages or?
Usually layers itself do not make a PSD big. A big PSD file is usually because the DPI is huge and >500mb PSD files are very common in the print industry where designers have typically design between 300 to 600 dpi. A B2 poster at 600dpi will easily be >500mb.
*A PSD file has a max height and width of 30,000 pixels, and a length limit of 3 Gigabytes. Hence, Photoshop created PSB filetype which supports max height and width of 300,000 pixels and the length limit to around 4 Exabytes. https://en.wikipedia.org/wiki/Adobe_Photoshop#File_format
To elaborate, for anyone who hasn't used Photoshop (in several years): you can have layer groups which act as folders containing several related layers, so when you're browsing layers, it isn't just a mess of 500 some-odd rows.
Sounds like what's needed is a way to extract a PSD into a component file structure, and combine that structure back into a PSD. Without knowing anything about PSDs, I assume it would contain lists of layers with links to resources such as images or whatever is required to approximate the desired view. Something like that as a plugin or hooked to run before and after commits would be really useful in this case.
Depending on how hard the PSD format is to grok (purposefully or otherwise), this may be nigh impossible, but it also seems like it would be a great exchange format (which may be a another reason Adobe wouldn't want it). Then again, maybe there's already something like this...?
*A PSD file has a max height and width of 30,000 pixels, and a length limit of 3 Gigabytes. Hence, Photoshop created PSB filetype which supports max height and width of 300,000 pixels and the length limit to around 4 Exabytes. https://en.wikipedia.org/wiki/Adobe_Photoshop#File_format