You can combine several of the codes from Config files with alternate the HTTP web-root
(e.g. HTTPD_ROOT=$HOME/src/webapp, nix-shell, loco init -f, loco run) and make one pithy variant:
$ git clone https://github.com/totten/locolamp
$ cd locolamp
$ env HTTPD_ROOT=$HOME/src/webapp nix-shell --command 'loco run -f -v'