Can I override the loading.. in each row?... NO

The loading message is shown for each row after any overlay. 

The life cycle of the loading message can be clearly seen in the plunker below when a two-second delay is added after the overlay is shown:

https://plnkr.co/edit/IuIfwpN6A9En8UPaDW9R?p=preview 

It is not possible to remove or replace the 'loading' message.

We have created a feature request that will allow this in the future:

AG-1049 - Allow loading row/column component