I suggest to not use a new text file format for that purpose.
The existing LDRAW format can combine two parts easily
AND position them correctly relative to one another:
1 16 ... ............ part1.dat
1 16 ... ............ part2.dat
That is what the shortcuts do.
As said, the issue is analogous to the physical color shortcuts:
there, instead of inventing a secondary database or format, we used the existing technique.
I like that approach of "keep it simple".
The existing LDRAW format can combine two parts easily
AND position them correctly relative to one another:
1 16 ... ............ part1.dat
1 16 ... ............ part2.dat
That is what the shortcuts do.
As said, the issue is analogous to the physical color shortcuts:
there, instead of inventing a secondary database or format, we used the existing technique.
I like that approach of "keep it simple".