I like this feature. Thanks for implementing it. I like both %o and %q.. not sure which one to pick yet.
I only have comments regarding the variable docstrings.
(1) I suggest adding more detail to the docstring of the new variable along these lines:
diff --git a/lisp/bindings.el b/lisp/bindings.el
index 0e6ffc275e..2a079000bf 100644
--- a/lisp/bindings.el
+++ b/lisp/bindings.el
@@ -366,10 +366,20 @@ column-number-indicator-zero-based
:version "26.1")
(defcustom mode-line-percent-position '(-3 "%p")
- "Specification of \"percentage offset\" of window through buffer
-This option specifies both the field width and the type of offset
-displayed in `mode-line-position', a component of the default
-`mode-line-format'."
+ "Specification of \"percentage offset\" of window through buffer.
+
+This option is a list where the first element specifies the field
+width to space-fill or truncate to, and the second element specifies
+the type of buffer offset to display in `mode-line-position'.
+
+The field width is specified as an integer. If the integer is
+negative (-N), the width is truncated to N characters, and if it is
+positive (N), padding is added if needed to make the field N
+characters wide.
+
+The buffer offset component can be one of \"%o\", \"%p\", \"%P\" or
+\"%q\". See `mode-line-format' for more information on these %
+constructs."
:type `(radio
(const :tag "nil: No offset is displayed" nil)
(const :tag "\"%o\": Proportion of \"travel\" of the window through the buffer"
--
(2) The docstring of mode-line-format also needs to be update to explain the new options %o and %q.