I used to have... I think it was called a FireCard? It was back in the days when the OG R4 was taking off, but it came with something like 1GB of built-in memory for the same price as an R4 without a microSD, and at the time it still would've been an extra $20 or so for a reputable 1GB microSD. It also took up both slots, with a USB adapter that plugged into the GBA slot and wrote to the DS slot, but didn't have any sort of ROM-booting or GBA functionality of its own. Pretty sure the GBA slot adapter was also sized to replace the DS Lite's GBA dust cover rather than being a full-sized cart, even though I swear this was only a few months after the Lite launched.