Adding all rows in a column

Hi,

yes, =SUM(A2:A123) is no problem.
And yes, using PHP so fill in the number should not be a problem. The best approach for that would be to hook into the tablepress_table_raw_render_data filter hook, which receives the full table (as a two-dimension array) as its parameter (see classes/class-render.php).

Note that this solution requires the current TablePress development version from http://tablepress.org/download/extension/tablepress-dev.zip , as this filter hook is not yet part of the current stable version.

Another solution, which is very similar, could to use the [table-info] Shortcode (explained a little bit in the documentation at http://tablepress.org/documentation/ ). In the development version, this has received another parameter, that allows to retrieve the number of rows of a table, see http://wordpress.org/support/topic/table-info-total-number-of-rows
If you combine that with the TablePress Extension that I briefly describe here http://wordpress.org/support/topic/calling-other-table-cell-values-from-another-table#post-3818970 , it should be possible to use a formula like

=SUM(A2:A[table-info id=123 field="number_rows" /])

inside a table (here table ID 123) that is loaded with the Shortcode

[table id=123 shortcodes_before_formulas=true /]

Regards,
Tobias