I wouldn't care about mounting anything read/write, or adding hijack hacks to update ext2 even when the filesystem is mounted readonly.

Just open("/dev/hda6", O_RDWR) (yes the swap partition), and you can just write blocks directly, add a checksum so that we can later tell whether the block was completely written or not. The player won't be using the swap in the car, and because the swap partition is as far as I can see on the outer tracks of the drive, I'm assuming it requires a smaller seek to park the head and in the worst case you'd only be trashing across the unused swap partition. Maybe even open it O_SYNC to keep actual writes as short as possible. There wouldn't be any metadata that could become inconsistent. I guess this is similar to how the player uses it's raw partition.

As long as we don't damage the first sector which contains the signature for the swap partition and recover the data when we're back on AC somewhere before the player starts an fsck.