You can use the function format-mode-line to compute the text
that would appear in a mode line or header line based on a certain
mode line construct.
This function formats a line of text according to format as if it
were generating the mode line for window, but it also returns the
text as a string. The argument window defaults to the selected
window. If buffer is non-nil, all the information used is
taken from buffer; by default, it comes from window’s
buffer.
The value string normally has text properties that correspond to the
faces, keymaps, etc., that the mode line would have. Any character for
which no face property is specified by format gets a
default value determined by face. If face is t, that
stands for either mode-line if window is selected,
otherwise mode-line-inactive. If face is nil or
omitted, that stands for the default face. If face is an integer,
the value returned by this function will have no text properties.
You can also specify other valid faces as the value of face.
If specified, that face provides the face property for characters
whose face is not specified by format.
Note that using mode-line, mode-line-inactive, or
header-line as face will actually redisplay the mode line
or the header line, respectively, using the current definitions of the
corresponding face, in addition to returning the formatted string.
(Other faces do not cause redisplay.)
For example, (format-mode-line header-line-format) returns the
text that would appear in the selected window’s header line (""
if it has no header line). (format-mode-line header-line-format
'header-line) returns the same text, with each character
carrying the face that it will have in the header line itself, and also
redraws the header line.