If you have high interest in such a feature, check out the NaviServer module nsconfigrw [1]. It provides support for changeability of configuration values and addresses scalability/performance via per-thread caching.
The module has a Tcl and a C-level interface. So if all you want is the Tcl-level access, it does pretty much what you want. The C-level code of the main NaviServer modules does not use this for obvious reasons, and as mentioned before, changing the configuration value is simple compared to handling the consequences of configuration changes (implementing the e.g. life migration semantics of a change of a driver/db-port, of an ip-address, change of modules, ...). So in order to make this really useful, one would have to add as well callbacks for value-changes that address such issues on a per-parameter basis.
[1] https://bitbucket.org/naviserver/nsconfigrw
