After the disaster with Rev. 4 (see the last log), I learned a couple of things and made some changes resulting in Rev. 5:
The mounting hole in the lower left corner of the PCB has been a pain since the start of the project, and if you look at the project logs, you can see it has moved around a lot. For Rev. 5 it's finally where it belongs: in the corner. I swapped the locations of the on/off switch and the power socket, so I could move the SRAM chip up a little, to make space for the hole. I can't believe I didn't think about it earlier and I don't think I'll have to move it again. That means it will be the location of the mounting hole on future expansion boards too.
That sounds pretty final, and it is. I think Rev. 5 is the design that will go into production. I adjusted some parameters (such as the pad-to-soldermask clearance, the flood plane clearance and minimum width, and the via drill and pad size), to prevent problems from Rev. 4 from reoccurring. I also nudged the keyboard connector over a little bit, to make it easier to prevent short circuits with the resistors when soldering.
Nevertheless, after the problems of Rev 4, I didn't feel confident enough to order a full production run of Rev. 5 at OSHPark. I placed a regular 3-board order and if they turn out okay, I'll order more.
I also ordered enough parts to have a small number of kits available on Tindie as soon as possible. But it looks like I may have to order a bigger batch of parts soon: There are already enough people on the waiting list to sell most kits of the first batch...
I updated the links to the OSHPark and Mouser order pages in case you want to order your own, but of course that will be more expensive than ordering from my Tindie page once it's ready. Thanks for your patience!
UPDATE: I got the Rev. 5 boards back from OSHPark and built one; see the updated picture at the top of the article. As you can see, the green headers are back!