I'd like to install Radiant heating this summer.  Two summers from now, I'd like to build an addition to the master bedroom.  How difficult would it be to "bolt on" another radiant heated section of the house?

Not difficult at all,
You would plan for it in several ways,
First a modulating boiler is a good start, it will have the capacity to expand somewhat as needed.
You will also want to accommodate in the boiler room the additional manifold on the new addition on the primary pump / zone valve manifold for the new zone, or you can oversize the loop manifold nearest the future bed room and leave extra circuits for future loops of pex. You will need a general plan for sizing your supply return lines, and an additional thermostat line will be needed.
Dan