The MS1 and MS2 use the exact same hardware for fuel injector drive, and they can both do PWM drive of Lo-Z injectors. On page 4 of the 3.0 the schematic, note the circuits consisting of Q9/Q12, Q10/Q13, and associated parts. Those are the PWM flyback clamps.
The difference, as Ben pointed out, is the Hi-Rez variant of the MS1Extra code cannot
do this, as the CPU resources which were (in the original builds) allocated to doing the PWM timing are instead hijacked and put to work doing the main timing.
However, there is a third solution, beyond merely running resistors or using the non-HR code. JBPerf, the same company that brought us the JimStim, sells a Peak&Hold Injector Driver Board for Megasquirt
that is compatible with all MSs. One cool thing is that this board does the P&H work in dedicated hardware, using a pair of LM1949 Injector Drive Controllers
. So you configure the MS as though it were driving hi-Z injectors, and let the board do the work. Another nice feature is that you get dedicated injector grounds just like a "real" ECU.