Landscape GUI hangs after unsuccessful "Add Hardware".

Unfortunately add-hardware can't recover from some failures yet and that activity will stay pending forever. That also means you cannot use the add-hardware feature again while it's in that state.

Is there any other way I could try to add a node to openstack? For example would this work: boot with MAAS, register with landscape and then use juju?

You could access the inner juju environment where this cloud is deployed and use "juju add-unit" for nova-compute, base-machine and ceph or swift. The new unit(s) would become landscape registered computers as usual, and I think even the cloud dashboard would account for them.

To access the inner environment, you first have to login on the landscape-server unit that deployed your cloud (that would be landscape-server/0 if you have only one unit). If you deployed landscape with juju, this command should do all the right things:

juju ssh landscape-server/0 sudo 'JUJU_HOME=/var/lib/landscape/juju-homes/`sudo ls -rt /var/lib/landscape/juju-homes/ | tail -1` sudo -u landscape -E bash'

It will land you in a shell as the landscape user, and with JUJU_HOME pointing at the environment where the cloud is deployed. Do a juju status there to confirm you can see all the cloud services. Then it's a matter of "juju add-unit" as explained before. Note that we install some services on the same node, something you can achieve also via "juju deploy <service> --to N" where "N" is the juju machine id (an integer).

You are in unsupported territory here, though, since you are changing the environment under landscape's feet. You shouldn't try this if you are not familiar with juju and OpenStack services.