Here's the point where it is all wired up.
Guess I hadn't considered that a 512GB SD card could be repackaged as a 32GB card if the wear pattern isn't even and only 90% of the cells are dead. Also, 2013. I'm not convinced recycling this way is cost-effective.
There is also AmazonSmile, although I am not 100% sure whether affiliate links still generate revenue when using a Smile link.
People in such situations should be encrypting anything sensitive that goes onto external storage anyway, you shouldn't be in a position where you have to "secure-erase" anything.
As for RAM encryption, Linux already supports it, if you're using a very new AMD CPU with SME: https://github.com/torvalds/linux/blob/master/Documentation/...
Relatedly, I remember the eyeFi (or similar) wifi-enabled SD cards got 'hacked' a few years ago, and you could get a Linux shell on them.
One immediate practical application is a hidden cryptographic partition on an SD card. It could be implemented in a way such that during normal use, no data shows up, and the corresponding blocks are marked as bad. But create a password file with the right content, and the firmware decrypts and exposes the hidden data. Since there's no way to read the firmware, only to update it, even detecting such a scheme would require tearing the SD card down in a lab - not really feasible for routine searches, given how many cards there are around (used in phones and tablets etc).