The code for topic_time(), used in topic listings, is here:
function topic_time( $args = '' ) {
$args = _bb_parse_time_function_args( $args );
$time = apply_filters( 'topic_time', get_topic_time( array('format' => 'mysql') + $args), $args );
echo _bb_time_function_return( $time, $args );
}
You can see that the filter topic_time filters the actual time, which is then passed to _bb_time_function_return(), which does a little processing and then passes it to bb_gmdate_i18n() which then returns something. A filter is not applied again (as far as I can tell - the time functions are not very easy to read).
If someone wishes to change how the time is displayed, they have to change the functions in their template files. In my opinion this would be better done with a plugin; it increases theme maintanability (since less theme files have to be changed, thus more of them can come from the up-to-date kakumei).
A plugin could be used or, equally, the change could be made using plugin-like code in the theme's functions.php. It may be a design decision, but I think it's best implemented using code, and for that bbPress needs to filter topic_time()'s output. The filter should also pass the time as an argument.
Additional filter names could be echo_topic_time and, for get_topic_time(), return_topic_time.
