dProg/dt in NAWIPS

Greetings gembuds,

I am a relatively new GEMPAK/NAWIPS user.  I have been able to figure
most things out, but I would like a little advice from a more
experienced user about a request from one of our operations forecasters.
They want to be able to view difference fields of MSLP between two
successive model runs.  This seems pretty straightforward to do in
GEMPAK directly, but not so straightforward for NAWIPS.  Specifically:

1.      Is it possible to build a restore file that can be put in
mod_res.tbl that does this directly?  If so, it isn't obvious to me how
one would set GDATTIM or GDFILE in those scripts, given that those are
somewhat tied to the input from the nmap2 data time selector.  To
compute the dProg/dt, I obviously need to load the MSLP field from the
previous run as well, and if the previous run is 6-hours older, then the
forecast time to subtract would be the current forecast time +6.

2.      Would I be better off writing some sort of GEMPAK script that
runs offline to create a new gridded data set that contains these
differences pre-computed, then adding this new data set to datatype.tbl
and mod_res.tbl?

Any advice from seasoned users would be appreciated!

Best regards,