Your idea doesn't work because it is based on faulty logic. Because you talk about waiting for a packet of 255, and using Framing errors I take it you are using some form of UART to read a character. To indicate the start of a character the data Rxd line has to be taken low to indicate a start bit. In DMX512 the first valid packet that is to be read after the MAB is the start code which currently has to have the value of zero. If you were using bit banging you could check that each bit after the break was 1 for a specified time. In your case you might find a value of 255 anywhere in the 512 channels so you would have offset the start of channel one randomly.