Unfortunately, it doesn't appear that it's possible to DMA write to the bitband region. Not only did my actual attempts all fail, but I think the documentation tells us that directly:
System 32-bit Address Range: 0x4200_0000–0x43FF_FFFF
Destination Slave: Aliased to AIPS and GPIO bitband
Access: Cortex-M4 core only
The "Access" section shows "All masters" and specifically "DMA" for other memory regions.