I would
1. put the function you want to manipulate in a separate .bmx
2. build your project with import "myfunc.bmx" once
3. copy the resulting myfunc.s produced in the .bmx folder back up to the main source folder
4. change your import "myfunc.bmx" to import "myfunc.s" and discard the original myfunc.bmx file
If you want to do something more complex use the verbose option when calling bmk from the command line so you can see how to build your own make script.
1. put the function you want to manipulate in a separate .bmx
2. build your project with import "myfunc.bmx" once
3. copy the resulting myfunc.s produced in the .bmx folder back up to the main source folder
4. change your import "myfunc.bmx" to import "myfunc.s" and discard the original myfunc.bmx file
If you want to do something more complex use the verbose option when calling bmk from the command line so you can see how to build your own make script.