On several occasions in long haul J, I asked the crew if they could help me cope with my food restrictions.
Over time I received a variety of responses, ranging from "please pre-order next time" to a memorable "here is the list of all the food we have onboard - including P - please let us know what you'd like".
Most often though, they have no clue how to manage food restrictions.
I'd say it's always worth trying, and it can be fun sometimes.