I tend to add a path to my :resource-paths in boot for dev-only things. You can do the same with lein (and hopefully any other build tool you use).
I'm not sure I got your question fully. My interpretation is that you want to have a different version of the "same" fn, so that other parts of the code that use the fn ends up using a diff version without needing to be changed. I think that's what you asked? :)
If so then adding a dev-only path like mentioned above, with the dev versions of the fns, and then e.g. add a wrapper macro that uses with-redefs to point the fns you want to the dev versions from the dev NS. You can then use this macro to to execute code with the dev versions, like (dev-mode (form1) (form2) ... ).