The quicklisp/local-projects/ directory is just one convenient place to put stuff, nothing dictates that you have to put it there.
You can also put your own .asd systems elsewhere and set up a call to:
(pushnew ".../my-projects/" ql:*local-project-directories* :test #'string-equal)(ql:register-local-projects)
Yes, you have to evaluate these two lines after making a new .asd file. But if you put all your projects under ".../my-projects/" then nothing else has to change so it can be part of a standard thingie you just call after setting up a new experimental project.