Yeah you can still sub. You have just described a monitored test since you used the word stall. A monitored test is used when the environment isn't under complete control such as a multi stall bathroom. Their presence is to prevent entrance of others into the bathroom or because the sinks can't be shut off, etc. They still can't observe you unless you give them cause and you have 4 minutes in the stall to give the sample. Use a quiet bottle and you can still pull it off.