I have DBML in my BAEC profile so they get ordered automatically.
In J you get the same number of courses as other pax. Normally the same salad. The main course is more often than not, one of stewed beef, chicken or salmon. The veg are boiled and you will normally get boiled potatoes or rice. I take bread from the regular bread basket - if you take a wholemeal roll healthwise it's certainly as good as the non-identifiable object you find on the tray. You get margarine and no butter and the desert is usually a fruit salad.
However that being said, usually at least one of the regular offerings is pretty diabetic compatible (maybe not the dessers!). I guess you could have a word with the cabin crew, explain the situation and ask them nicely to reserve what suits you best. For some time I have been thinking about removing the DBML label and making do with the regular offerings.
In summary the DBMLs are fine, not particularly inspiring and tend to get repetitive.
PS - you can of course sin and raid the club kitchen