>> But since a message is displayed at different part of the minibuffer,
>> it should be highlighted using a new separate customizable variable
>> similar to minibuffer-prompt-properties:
>
> 1. If you go that route, which lets libraries and
> users easily change the properties (good), please
> separate Isearch and `minibuffer-message'.  Use two
> different variables.  They are not the same species.
>
> The default value could be, but it need not be,
> the same for both.

OK, implemented in the following patch.

> On the other hand, it could be helpful to separate
> out property `face', defining an actual face (for
> each of Isearch and `minibuffer-message') that
> users can customize for this.  That also has the
> benefit that one can inherit from one to the other.
>
> The other properties would be applied first, so that
> if `face' is among them for some reason it will be
> overridden by the face provided specifically for this.

There are already too many faces in M-x list-faces-display
for users to comprehend that adding three more faces with
the same color would make matters worse.

> 2. The doc string of `minibuffer-message-properties'
> should not say that the properties are added to
> "minibuffer messages" - that's too broad.  It should
> say that they are added to the text shown by function
> `minibuffer-message' - or else describe that text,
> and only that text, in another way.

OK, fixed in the attached patch.

> The doc string of the similar variable for Isearch
> should say that the properties are added to "the
> search prompt" or some such.  Not sure whether the
> right description is "prompt", but whatever is the
> best way to describe it should be used in the doc
> string.

OK, added to this patch: