In First and Club, the special meals are served at the same time as regular ones, as it's all hand run anyway.

In WTP, it should be possible (but the request could very well be denied) to get your special served at the same time, but definitely tell the crew in advance so the trolley can be arranged.

In WT economy, it will be a big inconvenience for the crew as all the other special meals will have been delivered, as (as CWS mentions) the trays are kept separately, but the meals are also in a different oven. Some flights have up to 100 special meals, so asking for one to be kept back is the recipe for it to go missing.

When regular meals are being delivered, there is no way of guessing when a trolley will run out of trays (or hot meals) (there are only 26 trays on a trolley), so will be impossible to put the special meal aside in the top of the trolley supposedly heading your way.

All that being said, most items on the tray are cold, and the hot element will probably keep pretty warm for a while with the foil on. Depending on where you're sitting, the wait shouldn't be too long for the regular meals to catch up.