u gotta get somebody that really wants to be in unison with you and help you (and others if there are others) to get there. our organist is always on his own so i dont even bother too much and i just leave it alone. but the thing is, once u get teased with a positive musician, you kinda get spoiled. for me, the keyboardist that first started teaching me bass came up to the organ to play during an afternoon service because our organist didnt show. and when the MC started going into that preach mode, kinda ministering to the people, he leaned over and asked me "what key is he in?" im like "what!?!" he tells me, the MC is speaking/talking in a key; find it. so i start fiddling on my bass with it turned down kinda low and i found a note that kinda felt right. and i said "is it G?" and he says yeah. (A entire lesson right there on the spot, because i never figured that people talk in a key and i wouldnt have ever learned that if he didnt tell me to do that) so he starts playing a progression in G and tells me the chords as hes playing, so once i get the pattern down, i know the actual chords so i can try different stuff to fill in if i choose too (im not sure our organist can even tell you if hes playing a Cm7, and if he bothers to even tell you what he's playing he seems to only spell out the notes C,Eb,G,Bb which takes way too long when the song is movin along)
so i say best bet is to first get with a good organist/keyboardist to work out some progressions to use during those times. so at least you'll be able to identify some patterns and practice them to figure out stuff that works and stuff that doesnt. then if you can, get another bass players idea of what can be worked within those constructs