I found docs about profiling settings in mongo. I think I might have already seen references to these but dismissed them as not what I was looking for because I had been focussed on the word "logging". This is what I did:
db.setProfilingLevel(2)
// Then ran a mongo command through my app.
db.system.profile.find()
I inspected the profiling for a use of mgo.Pipe. I could see the pipeline stages data structure as it would be entered in the Mongo console. This let me easily cut-and-paste it into a command in the console. Problem solved.
Once you have a few kinds of entry in the system.profile collection, it may be useful to filter them to find what you want, e.g.
db.system.profile.find({"ns":"myDbName.MyCollectionName"}).pretty()
(I guess "ns" stands for namespace.)