Hi August.
1.) since pd is based on tcl/tk, would it be possible (easy?) to impliment the tcl scripting language inside of PD?
it's possible for sure - it's a task comparable to my py/pyext-externals. You may have a look at them at http://www.parasitaere-kapazitaeten.net/ext
2.) when externals and/or libraries are included in the pd path (say in the .pdrc file) are they loaded into entirely into memory or just registered somehow? I have the feeling that librarys are loaded, but externals are not.
This is right. The setup functions (which registers the classes along with their methods) of these external files are loaded either at pd startup (with the -lib command) or at instantiation of an object (the external is then searched via -path). Once loaded the information stays in the system. It's not precise to say that they are "loaded entirely into memory" because memory consumption will in most cases take place when an object is created, not upon loading (which is "registering") a library or single external. So, (both for libraries or single externals) you can choose to "-lib" them at startup or load them on demand by creating an object (a library can also be loaded by creating an object with the library name).
best greetings, Thomas