For the substitution of links extlinks is fine, for including arbitrary config values as asked in your question you can use rst_epilog for substitutions (or rst_prolog for text, that should be added on top of your .rst files):
In your conf.py:
my_config_value = 42
rst_epilog = '.. |my_conf_val| replace:: %d' % my_config_value
In your .rst source:
My config value is |my_conf_val|!
In your output:
My config value is 42!