As I can understand we need to pass new tab creation argument to the Meld.
According to its man-page this feature exists in Ubuntu 16.04 LTS (see man meld) and above:
NAME
meld - Visual diff and merge tool for the GNOME Desktop
...
OPTIONS
...
--newtab, -n
Open the comparison as a new tab in an already-running instance.
So we need to pass --newtab option to meld:
meld --newtab /etc/os-release /etc/os-release &
meld --newtab /etc/os-release /etc/os-release &
In Bazaar case we need to find corresponding option in bzr diff subcommand.
According to man bzr we should use --diff-options option:
NAME
bzr - Bazaar next-generation distributed version control
COMMAND REFERENCE
bzr diff [FILE...]
...
--diff-options ARG Pass these options to the external diff program.
Possible solution:
bzr diff --using meld --diff-options --newtab &
bzr diff --using meld --diff-options --newtab &
But in real situation as stated by OP it does not work.
So we need to create an alias for this command:
for f in $( eval "bzr stat --versioned" ); \
do eval "bzr diff --using meld --diff-options --newtab ${f} &"; \
done