Avalon ST custom IP component

rockoff, if I understand correctly, you want to have a component that performs some computations on the pixel values so you want to avoid changing the control packets, right? If yes, then you can detect the start of packet input signal and then check the least for bits of data input. if they're "0000" then the next packet that will arrive is the pixel value. To give you an example:

 if DIN_SOP = '1' then
	if DIN_DATA(3 downto 0) ="0000" then
	      --set some signal to know that you're dealing with pixels next
                 data_pkt <= '1';