I have to agree with offload
When I worked for a handling agt I had to follow the rules set up by the individual carrier we handled.
The carriers often have rules for upgrading: first goldcardholders, then fullfare y pax, then this, then that. ID N2 tickets are in the end of that list.
All of them had one basic rule: don't order extra catering for complementary upgrades. Its too expensive. The airlines pay not only for each meal, but also for every extra delivery.
I have found this to be normal at "away stations", while at the carriers bases/hubs, they have plenty of meals, and they are not worried about ordering axtras.
When you are at a gate where the ac is almost full, chances are that the workload at the gate is high, the staff have to check meals, late transiting pax, w/b so they don't want to be disturbed by a non-revenue for an upgrade. If an upgrade is needed or available, they will contact you.
So it all comes down to:
the availability on the flt (in J cls)
the people you know
the goodwill of the gatestaff
I have to admit I also enjoy chasing upgrades myself when flying N2/R2. Good luck