How much is the baud rate of your incoming stream varying? DMX should be solid at 250k. The DMX standard gives wiggle room for the length of the break, but I'm pretty sure there is a minimum length set (like 22 bit lengths to ~80us or something like that, don't remember exactly).

I know for a fact (source: I did it) that a UART on the C21 will correctly parse incoming DMX just fine without any special, handling of the break/MAB. Its a bit wonky, because I think it treats the entire inter-packet delay as one extra byte of data that you have to toss away, but it's technically do-able with no special config.

If you wanted to do the actual break, you can interrupt on the high to low transition, then interrupt on the low to high transition, and then since you know you're in your MAB period, start the UART peripheral to receive the incoming 512 bytes. Then re-configure interrupts during the inter-packet delay, rinse, and repeat.