Perhaps they could sing before the choral preludes that you want to play or around Christmas, you could perform some carols (with singing). It seems like the singing of the hymns and chorals would be a good introduction to the organ preludes (and fantasias and variations).
So you did all the preparations (including the booklet with the specific indications of when and what the people should sing). You planned a heavy registration because the church might be full and the singing should be quite loud.
At the night of the recital you notice that the church is really full (not only your friends and family members showed up but also the members of the congregation, too). This is good news.
You play a few organ pieces and everything goes according to plan. But when the time comes for the people to join the singing, you find out a few things:
1. People are reluctant to sing.
2. The organ is too loud.
3. You have to make stop changes which you didn't prepare for.
4. You mess up the unexpected registration changes.
5. People sing not in tune.
6. All of the above make you very nervous.
7. Therefore, you start making silly mistakes.
8. Somehow you manage to recover.
9. After the recital the feeling of failure is quite pronounced.
10. You feel embarrassed.
11. You swear not to play with hymn singing ever again.
12. You meet the people after the recital and they seemed to enjoy the singing and your performance.
13. Most importantly, they ask you when your next concert is going to be.
1. How did it happen that people didn't sing very well?
2. How could it be that even because of the poor result (in your opinion) they seemed to enjoy and want for more?
3. What can you do differently next time so that the singing (and the playing) would go better?
You see, when people go to organ recitals, they don't expect they are going to sing. Their mindset is different. They want to be entertained or simply listen to the music while sitting alone with their thoughts. Unless you announce ahead of time the new rules of the game, people may be quite surprised, at least at first.
But in the end, if the tunes are familiar to them and if they come from the culture where singing in public is a natural phenomenon (in many countries this is no longer the case), I think that the singing towards the end of the event can actually become more and more pronounced and in tune because the audience will adjust their expectations.
What can you do differently next time? On the poster, press releases and other promotional material you could write really clearly that there will be some singing involved. Indicate clearly the same information on the program that you hand out to them.
Practice registration changes (even the unexpected ones) ahead of time. If the organ is unfamiliar to you, investigate it's specification, placement of stops by contacting the local organist. Once you know what to expect, imagine its sound and imitate the stop changes on your own instrument.
This practice will calm you down and you will feel a lot less anxiety and make fewer mistakes when the time comes for your next recital.