Just browsing through the available options, it looks like the nova meta actions may be available as openstack server set:
$ openstack help server set
usage: openstack server set [-h] [--name <new-name>] [--root-password]
[--property <key=value>] [--state <state>]
<server>
Set server properties
positional arguments:
<server> Server (name or ID)
optional arguments:
-h, --help show this help message and exit
--name <new-name> New server name
--root-password Set new root password (interactive only)
--property <key=value>
Property to add/change for this server (repeat option
to set multiple properties)
--state <state> New server state (valid value: active, error)
E.g., if I run:
openstack server set myserver --property foo=bar
And then run:
openstack server show myserver
I see:
[...]
| properties | foo='bar' |
[...]