Is there a way to embed comments in an Instiki page which appear only in the source itself and not to a casual reader?

Short answer: no.

You can try embedding (X)HTML comments in the source, but they get escaped (for a variety of security-related reasons) and hence are “visible” on the page.

Longer answer: Metadata about the page can be placed in a way that does not appear in the rendered output. The syntax is that of email headers (key-value pairs, at the beginning of the page, separated from the main text by a newline):

Author: Me
Subject: My personal musings
Blortnaz: Whatever you want to put here

And now begins the main text ...

This is not quite what you asked for, but it may serve your purpose, depending on what use-case you had in mind.