Answering my own; well, confusion arose because 'help-echo is not a keyword argument as in:

(defun foo (&key arg1 (arg2 "x"))

but instead a symbol to be matched later in internal emacs text properties code.

Harvesting the docs from insert-button into text properties:

"...Each property has a name and a value. Both of these can be any Lisp object, but the name is normally a symbol...."

and these are properties with special meanings: special-properties