The only thing I can think of is to
run the subreport in GHa
suppress all of the fields in the sub report,
set the sub report to suppress if blank
set GHa to suppress if blank
return all of your fields as shared variables in a GHb
suppress them all
set GHb to suppress if blank
make your 'current gh' as GHc
now you can conditionally suppress GHc, details and GF on the shared variables (returned in GHb) as all = 0
palce the shared variables in you GF as you you have now to dispalyt he values you want.
Note I am purposefully trying to only run the subreport once per group to limit its impact on the report performance