Well, a few pertinent facts:
Raspberries (or Rubus idaeus) have "drupelets", and each drupelet has a seed inside, so raspberries indeed have seeds, (not a singular "seed") and not like strawberries "pips" on the outside (or rather in-between) their drupelets.
The raspberry is a type of plant that has "basal shoots", that is, it can form a new plant by growing a shoot that roots at the end, and forms a new plant. Maybe "basal shoot" (AKA as "adventitious shoot" or "sucker") could be the basis for the name for the "rooted expansion boards". Although "sucker" might not be a very good name
I really like the name "crust", as synonymous to the arduino's "shields", (for the obvious reason that both protect the softer stuff below) but it proposes we pretend that "PI" stands for "PIE", I'm not sure that is really true, or that PI simply refers to the mathematical constant, or as some people claim is alluring to the Python language.
If you look at the R-PI logo its obvious that "what covers the Raspberry fruit" is indeed a pair of leaves.