I'm not sure I'm following this. The "Radiation" units in MTCLIM are "W/m2, averaged over daylight period" (case1). Note that daylight hours change throughout the year. The radiation units in my input file are W/m2, averaged over the day period, that is over 24 hours (case 2). 
The function I modified sends radiation from MTCLIM back to VIC. Originally it multiplies the Radiation (assuming case 1)  by the daylight hours for each day of the year to estimate the total amount of radiation in a day, leading to an underestimate of daily radiation when radiation is read from an input file and correspond to case 2. Note that the units of daily radiation are W*hour/m2.
This is why I create an if statement in the function, so that if radiation is internally computed, it calculates daily radiation following case 1, and if radiation is read from a file, it calculates daily radiation following case 2.
Fixing this outside MTCLIM seems more complicated and less clear. In terms of compatibility with MTCLIM, note that the fix is in the wrapper, not in MTCLIM itself.