I think Mprint disable the HBP internally and filter out all HBP command while it generating the g-code and .x3g file. Edit the Mprint machine & filament profile or even edit my own start/end g-code with HBP setting, but the g-code generated by Mprint (OnlinePrint.gcode) is still filtered out.
But there's still a work-around.
Indeed, the .x3g generated from the post-editing Mprint g-code by RepG would crash the printing. I think Mprint actually use "GPX" as the g-code to x3g converter, which could be found under the "tools" folder of Mprint installation path, also the configuration file specific for MBot series printer (gpx.ini). Somewhere in gpx.ini define a variable "has_heated_build_platform=0" to filter the HBP command, just change this the value to 1 to let it compatible with HBP. Generate the .x3g with your post-editing g-code at command-line under this folder with command like "gpx.exe -x gpx.ini OnlinePrint.gcode", it should generate x3g without warning and make your printing not crash due to HBP.