In case anyone else wants to send raw HEX via the serial binding, here is how I finally got it to work after adding the environment variable mentioned in my last reply. If you know of a better way please let me know 
You will notice some of it is from the DEMO. This will send a hex string containing the value from the slider/fader/dimmer.
My item file contains:
String RS485Bus “RS485Bus [%x]” (All) {serial=“COM8@115200”} //serial binding
Dimmer RS485Bus_Light_91_02 “Ceiling [%d %%]” (Bathroom, Lights) // creates the dimmer
My rules contain:
rule “RS485Bus_Light_91_02”
when
Item RS485Bus_Light_91_02 received command
then
var Number percent
if(RS485Bus_Light_91_02.state instanceof DecimalType) percent = RS485Bus_Light_91_02.state as DecimalType
if(receivedCommand==INCREASE) percent = percent + 5
if(receivedCommand==DECREASE) percent = percent - 5
if(percent<0) percent = 0
if(percent>100) percent = 100
postUpdate(RS485Bus_Light_91_02, percent);
//Convert 0-100 range into 8bit hexadecimal.
val Number scale = Math::floor( percent.floatValue * 2.55).intValue
if(percent==0)
{sendCommand(RS485Bus,'\u0091\u0002\u0001\u0008\u0000\u0010\u0020\u0002')}
else if(percent==100)
{sendCommand(RS485Bus,'\u0091\u0002\u0001\u0008\u00FF\u0010\u0020\u0002')}
else
{sendCommand(RS485Bus,'\u0091\u0002\u0001\u0008'+ String::format("%c", scale) +'\u0010\u0020\u0002')}
end