"Harmonizing" the :height attribute for multiple faces in the same string?

[Bastien]

Dear all,

given a string combining various substrings each with a different
face, each face with a different :height attribute: how can I set
:height to the same value for all faces in this string, keeping 
the faces (and their attributes) distincts ?

The use case is this: when navigating Org headings, the minibuffer
displays the heading the point is on, but when users customize the
height of the various heading levels (org-level-1, etc.), then the
displayed heading in the minibuffer combines multiple heights,
which is ugly.

Any help appreciated!

Thanks,

-- 
 Bastien