yes,, of course we can change scale and send it to modelor then adjust detail and save it as morph target,
or can use D-form too.. (or may need to adjust rig if change much )
I need to say one more thing,, ,, if just change head node scale ,other joe and teeth came from skin ^^;
so I think better select all childeren of head first, with head node, then change every child bone scale with head scale.
it must work better,, ^^;
today I checked again my step. because I made some head morph by scalling head node,
when check this topic, I planned to make cotroller which can easy scale every head and childere bone at same time..
but,,there is problem still, x scale change, seems no problem, but if you change only Y scale, or Z scale,
the tongue and teeth positon may change hard, and these rig positon not good too.
when open mouce, you can see strange face.
if use this shape as morph,,,, I may need to export them,,
then adjust positon and scale about the teeth, tongue and lower joe by modelor,
then re import it as morph taget,,, after that,,, need to adjust rig by joint editor. or Adjust rigging to shape,,.
it seems not easy,, if you have not used these step,,
so I think,, if you change head scale (must need select children nodes at same time ) by node scale,,
better not change the raito Y scale and z scale.
keeping Y scale and Z scale as same ratio, there seems no problem.
eg head and children node change Y scale 150 Z scale 150, can work (you can change as you like X scale)
but Y scale 120 Z scale 150 or Y 150 z 120, cause terrible shape.how you adjust X scale.
so need to keep ratio of Y scale and Z scale,, I think.