Based on the results of the ensuing discussion, I decided to 
refine my macro to just be one coherent defun.  I experimented 
with a few different ways to do this, but settled on something 
that seemed relatively clear and minimal, with pattern matchers 
that seemed like a better fit for arglists than what exists in 
pcase already (but it's all pcase under the hood).

To give a flavor of what it looks like in practice, here's a few 
pattern defuns from the test:

(defun-pattern fibonacci
    "Compute the fibonacci sequence."
     ((0) 0)
     ((1) 1)
     ((n)
       (+ (fibonacci (- n 1))
            (fibonacci (- n 2)))))

(defun-pattern crit
  "D&D 5e critical hit computation, taking in the results of a d20
and a list of attributes currently in effect.  Returns whether the
results are a crit or not."
   ((20 _) t)
   ((19 (pred (member 'improved-crit))) t))

I'm attaching the package, which works, although there's some 
non-ideal parts of the implementation.  If people think it is 
valuable, I can add this to GNU ELPA.