If you are asking about diagrams, I have never seen a field transmitter represented on a diagram as a resistor. There is the ANSI/ISA 5.1-20009 spec (NEMA or IEC symbols are switches, not transmitters) with symbols and labels that represent the function of a field transmitter, like flow transmitter FT or pressure transmitter PT.
The analog input resistance used for current loops in DCS systems is usually 250 Ohms but that resistance does not 'represent' the field transmitter.
HART operates as FSK technology using two audio frequencies, 1200 Hz or 2400 Hz, which is where your 1200 number might have come from, but no one would typically recognize the number 1200 as meaning HART.
I'm not sure what you're trying to accomplish but if designating HART were a major consideration, I'd write the word HART and move on.
If you are asking about substituting a resistor for an actual field transmitter, the answer is no, a resistor does not do what a field transmitter does. Substituting a resistor for field transmitter would provide the analog input a constant current, but a constant current is not a signal proportional to a measured variable.
