Your idea is already here in the forum: www.rainlendar.net/cms/index.php?option=com_kunena&Itemid=42&func=view&catid=6&id=13288#13288
All calendars are drawed as images. Changing an image for 28, 29, 30 or 31 buttons looks like a more complicated thing. (I think)
Another thing is the impossibility to change the displayed day by script, because the function that does that doesn't allow to change the day, only month and year (day is fixed to 1).
For the "goto function", I've made a workaround for quick changing month and year. You can found it here:
this skin will change the displayed date on the grid calendar and month view among others windows. But WON'T change the daily view (for the above reason) and the weekly view (because don't use the date, only week number)