We have been overzealous and purged the P&ID Class Definition for the "Front Facing Nozzle" in our main project.
Is there a way to get it back into the project without loosing our other customised class definitions?
Failing that, is it possible to implement a new nozzle subclass that can be placed anywhere inside-a-component just like the Front Facing Nozzle, rather than on-a-component like the other nozzle classes?
I don't know of any way to bring it back other than reverting to a backed up version of your project folder and possibly even SQL/Lite backup. If you are unable to recover it from that, you may just have to create a new one. I find that all nozzles prompt you to place them on a piece of equipment before you can move them inside of the equipment component anyway, so you sould be able to make a new block, or borrow the front facing nozzle block from the default project's projsymbolstyles.dwg file.
Also, as learning this the hardway myself, be very careful when purging nozzles classes. I can't remember which (maybe the regular flanged nozzle), but if you purge just the right one that holds the auto-nozzle feature, you may lose that feature altogether as I did in my very first project setup. Once purged, there's no way to get it back. See this post for more info of what I did wrong: http://forums.autodesk.com/t5/AutoCAD-P-ID/Auto-No
I hope this helps!