tested with a 2GB silicondriveII CF card, recorded in WSD till full, played back smoothly. sustained write speed is 6MB/s according to the data sheet, more than enough for the ~672kB/s of stereo DSD64. card is ATA-5 and i have a feeling success in running this mod has more to do with ATA standards than random write speeds or SLC/MLC. will try again with a higher capacity – this card only holds about 48 minutes in 1-bit.
update 16.03.24 – tested a 32GB sandisk ultra 50MB/s CF card with a two-hour WSD recording. recording went well, closing the file went well, playback went well. i'm satisfied that this works. using a generic IDE ZIF adapter bought over amazon.
between this CF card replacement and an earlier battery replacement (EEMB 963450), this recorder feels new and modern again. nothing else in its size that i've seen has balanced inputs, so even imagining a replacement is a tall order even before taking into account binary recording. opening it up i was surprised and impressed by the RF shielding inside. it's a shame the included mic isn't shielded at all.
i owe everyone in this thread a debt of gratitude. i wouldn't have attempted either mod without you all.