I'm not finished yet, but I think I found a PHP-based solution, which is fine since I already use that.
Within the MTCalendar tags, I would modify the code to look something like this:
CODE
<td align="center" class="calendar">
<span class="calendar<MTCalendarIfToday>today</MTCalendarIfToday>">
<MTCalendarIfEntries>
<?php
$dailyCt = 0;
<MTEntries>
$dailyCt++;
</MTEntries>
?>
<MTEntries lastn="1">
<a href="<$MTEntryLink$>#<$MTCalendarDate format="%d"$>" title="<?php echo $dailyCt;?>"><$MTCalendarDay$></a>
</MTEntries>
</MTCalendarIfEntries>
<MTCalendarIfNoEntries><;$MTCalendarDay$></MTCalendarIfNoEntries>
<MTCalendarIfBlank> </MTCalendarIfBlank>
</span></td>
Basically, use PHP to count the entries for that day, then later use that count in the link title. (It can't be combined with the second MTEntries tag because it uses lastn="1" to generate just one link; for more than one entry per day there would be multiple links generated. However, the PHP code to count entries would always be 1 if it were placed inside that MTEntries tag.)