There is a much easier solution to this problem. Build your client
with the required heap space
it requires, and you won't have to fiddle with LDR_CNTRL.

xlc_r -bmaxdata=0x80000000 ...

I don't think you're going to be able to achieve this with putenv.
It's a matter of environmental scope ... you've set the variable into
the main environment,
but it isn't "export"ed for use in the child.

At least that's my experience. I just gave up trying to do that.