So, I was rummaging around on the table (which looks like a bomb hit it… yes I have hoarding tendencies) and I managed to find the packet of MOSFETs I originally used to make the earlier prototype I/O board.
These are Toshiba 2SK4017 s, probably not the best pick but at 70c a pop, they were cheap. They worked for the original version, and at 5A drain current, are more than sufficient for the task.
Above is the re-built board. I literally cut away the old MOSFETs (hey, should still be able to dead-bug them, but sod it, I’ve got 42 of them left), cleaned up the board a little, inserted these smaller MOSFETs, and soldered them in. I had to replace two 0805 1k resistors which got damaged due to the end of the resistor overlapping the hole for the gate… but whatever, I’ve got nearly 5000 of those kicking around too.
In short, it works. I didn’t try it with full power, but rather used a series resistor with 9V, but I could light up a button and detect a press on every single channel.
I’ll have to order some more MOSFETs. Not sure whether to go with these same ones again, I think they’re obsolete … but now I have a couple of datasheets to compare against since I now know what does work, and what doesn’t .