I ended up with the code below. I guess it can be shorten/optimized (it's a bit slow) but it does the job.
(defface busy-1 '((t :foreground "black" :background "#eceff1")) "")
(defface busy-2 '((t :foreground "black" :background "#cfd8dc")) "")
(defface busy-3 '((t :foreground "black" :background "#b0bec5")) "")
(defface busy-4 '((t :foreground "black" :background "#90a4ae")) "")
(defface busy-5 '((t :foreground "white" :background "#78909c")) "")
(defface busy-6 '((t :foreground "white" :background "#607d8b")) "")
(defface busy-7 '((t :foreground "white" :background "#546e7a")) "")
(defface busy-8 '((t :foreground "white" :background "#455a64")) "")
(defface busy-9 '((t :foreground "white" :background "#37474f")) "")
(defface busy-10 '((t :foreground "white" :background "#263238")) "")
(defadvice calendar-generate-month
(after highlight-weekend-days (month year indent) activate)
"Highlight weekend days"
(dotimes (i 31)
(let ((date (list month (1+ i) year))
(count (length (org-agenda-get-day-entries
"~/Documents/org/agenda.org" (list month (1+ i) year)))))
(cond ((= count 0) ())
((= count 1) (calendar-mark-visible-date date 'busy-1))
((= count 2) (calendar-mark-visible-date date 'busy-2))
((= count 3) (calendar-mark-visible-date date 'busy-3))
((= count 4) (calendar-mark-visible-date date 'busy-4))
((= count 5) (calendar-mark-visible-date date 'busy-5))
((= count 6) (calendar-mark-visible-date date 'busy-6))
((= count 7) (calendar-mark-visible-date date 'busy-7))
((= count 8) (calendar-mark-visible-date date 'busy-8))
((= count 9) (calendar-mark-visible-date date 'busy-9))
(t (calendar-mark-visible-date date 'busy-10)))
)))

lorg-calendar
which includes some highlighting onto a 3-month or 12-month calendar for certain events: github.com/lawlist/lorg-calendar And, as I mentioned in one of my previous comments to a thread you linked in the question above, one idea is to extract the data that is used to create the org-agenda and operate on that extracted data to generate the calendar view:emacs.stackexchange.com/a/12563/2287lorg-calendar
repository example linked in the previous comment, I use a modified version of amark-visible-calendar-date
by Drew Adams in hiscalendar+.el
library: emacswiki.org/emacs/calendar%2B.el That library may have some other goodies that could help you on your quest.