Well I managed to make something work with the ExportHelper.invoke
source...
This is coming from not knowing about the invoke
method because the example doesn't provide it let alone explain where to look for more info.
what I did to access the data was simply to use the invoke method and integrate and modify the code from ExportHelper
into my class rather than inherit from it.
to supply the needed data, all I did was create a dedicated PropertyGroup
initialized in the scene context, and when invoke
was called, clear and update the CollectionProperty
items with their associated index IntProperty
items in the group.
there may be a better method than this, but it works for me:

I did the dialog with this at the end of invoke
before returning {'RUNNING_MODAL'}
:
context.window_manager.invoke_props_dialog( this, width=900 )
Would be nice if blender's api was documented as well as they claim it to be...