Does the Zero allow DMA from a non-local device, like another MCU?
Yes, insofar as it's possible to set up the DMA to perform I2C, SPI and serial transfers.
The DMA can perform not only memory transfers, but also read and write to peripheral registers and be triggered by peripheral interrupts. This includes the ADC, DAC, SERCOM modules and TCC/TC timers.
The DMA is really useful for any situation that involves moving data from memory to memory, peripheral to memory and memory to peripheral without the need for processor intervention. The transfer occurs in parallel with the code in your sketch.
In addition, on the Zero you've got 12 DMA channels with an arbiter to control transfer priority, so it's possible to kick-off more than one transfer at a time.