I'm not sure I understand what you mean in the first part, if you mean having a option not to add a registration, that becomes a little tricky with FSACARS, and other statistics functions which are coming.
But when filling a PIREP, it's not restricted to the assigned aircraft. It'll take whatever registration they enter (in FSACARS or on the form), as long as its been added. I suppose you can add an "aircraft" called none, with "none" as the registration and then assign that to your schedules. The stats will look at the PIREPS as the reference point on usage stats, so there are no worries about "holes" in the actual usage.