Dear Erik,
Thanks for that very clear steer that the WMF is willing to help others build a mass upload tool for commons.
Two small points. Whilst many potential users of a mass upload tool are GLAMs, there are other opportunities out there for mass uploads, and I think it would be helpful if we remembered that and called it a mass upload tool that the GLAM community is the strongest supporter of rather than just a GLAM tool. The ability to upload millions of files while mapping other people's metadata into ours is important because it saves on volunteer time in uploading and categorising images in other ways. Commons has huge backlogs of uncategorised or poorly categorised files, and other projects especially wikipedia have vast backlogs in adding or upgrading their imagery. There has been recent research that articles with images get more viewers, so the implications of backing this aren't just the usual one that investing in the GLAM program is an investment in quality, there is also a good chance that investing in GLAM could increase readership.