Hi,
I suppose that with Class 900, expansion joints are not an option.
As you explain, and based on NEMA SM-23 (and API 617) the reference point is the center of the biggest flange.
NEMA is a turbine code, it states loads are to be resolved at "centrelines of the exhaust connection", the centre of the biggest nozzle is chosen as resolution point.
It can be difficult to reach 3 times NEMA, but if your process guys let you put elbows which add lots of flexibility you can reduce forces and moments. Try to block the movements of the piping in the same planes as the fixed feet of the equipment.
You can also be precise in your modeling: instead of "infinitely" rigid supports, try to take into account their rigidity. The same applies to the compressor or at least the supports of the compressor.
With CII beam modeling possiblities, this is an option I would try for such a case.
Remark: I would say that NORSOK _requires_ 4 times NEMA from the equipment manufacturer, not "allows"