Specify a custom config file for obnam
Untested by me. From the man page:
Configuration files and settings
--config=FILE
add FILE to config files
--dump-config
write out the entire current configuration
--dump-setting-names
SUPPRESSHELP
--help-all
show all options
--list-config-files
SUPPRESSHELP
--no-default-configs
clear list of configuration files to read
Which ought to mean that this should work:
obnam backup $HOME --no-default-configs --config=FILE