ok, that makes it more clear.
I just had a quick look at the code again, to see where to hook in for your idea.
And I come to the conclusion that using the "wp_table_reloaded_cell_content" hook actually is the correct place.
Besides the content of the cell, it gets also passed the table ID and the row and column number.
With that you should be able to determine if you want the row to be spanning all cells.
Then, all your function needs to do is return "#colspan#" as the actual search for that value will then colspan that row.
So, basically, your current function that is hooked to the filter just needs to return your own content for the first cell of the row. For all other cells in that row, it needs to return "#colspan#". Does that make sense?
I don't see why this should not work for you, but just in case: Two other possibilities would be post processing as you mention it (which I don't really recommend), using the "wp_table_reloaded_post_output_table" hook, or filtering the table content upon loading (hook "wp_table_reloaded_post_load_table" (include a check for is_admin() in it!). That would give you the array of the table values where you could do your necessary content insertion, also using the "#colspan#" words in the cells. That way, the actual rendering would still be done by WP-Table Reloaded.
But after thinking about it again, the "wp_table_reloaded_cell_content" really would be the best approach.