Robert Rohde wrote:
We already have {{formatnum:}} with a very limited functionality that presumably could be extended.
Though I would like to re-emphasize that Greg's complaint principally arrises because of floating point round-off errors in #expr that are difficult for normal editors to predict or plan for, and that should be addressed irrespective of other work to improve number formatting.
-Robert Rohde
The problem doesn't lie on rouding errors per se, but that they're using math functions for string actions, because mediawiki doesn't provide anything better.