Sending status messages to program message queue using QMHSNDPM

I have an ILE program running that performs variable amounts of work over a
number of libraries and want to send progress messages to a program message
queue while the job is running.

The setup I have is:

PGM_A consists of two modules MOD_1 and MOD_2 and two service
programs SRV_1 and SRV_2

MOD_1 has a display file defined which includes a message subfile

MOD_1 calls MOD_2 to do the work

MOD_2 calls sub-procedure PROGRESS which is found in SRV_1

Sub-procedure PROGRESS in turn calls the SNDPGMMSG sub-procedure
which is in SRV_2

Sub-procedure SNDPGMMSG calls the QMHSNDPM system API

I have tried various combinations of setting the call stack entry parameter
for the API to "*" and "*EXT" as well as passing in the program, object and
module, and the most success I have is passing in the appropriate values for
PGM_A and MOD_1. However, the messages don't appear on the display until
the process completes and the message subfile control is rewritten.

Does anybody know if it is possible to have the status messages pop up on
the display while the job is running and what I need to do to achieve it.

Thanks

Jonathan