Sorry for what has turned out to be a fairly long reply!
HelipadR22:
Last time I checked LASORS on the subject, ISTR that you can count up to 10% of your TMG PIC time (up to a max. 10 hours) towards the JAR PPL hours requirements.
You can also count up to 30 hours of TMG PIC time towards the CPL hours requirements, BUT (oddly enough) you cannot use it to satisfy the 100 hours PIC requirement which is part of it, only the total time aspect.
Note that it must be PIC time on motor gliders, not any "kind" of time...
And back to the OP:
As previous posters have discussed, the only way I can see this working is through some sort of NPPL SLMG - SEP conversion - JAR PPL conversion route. I guess it might save a bit of money through the difference in hourly rates etc.
HOWEVER, as the OP is specifically looking to move towards a commercial licence, he may lose out when it comes to hours building later on. As per my understanding of LASORS above, all of that dual time on motor gliders during his PPL training could not be counted towards his hours building, leaving perhaps an extra 20-30+ hours of SEP flying that he would have to pay for at a later date in order to reach the magic 150. But if he did his whole PPL on an SEP type instead, most if not all of those dual hours would count towards it.
Perhaps, as others have already suggested, a false economy?
LAI (standing by to be ripped apart by someone who actually knows what they're talking about
)