Network-Manager has a lot of features. You can customize dhcp client-hostname or any other option for every network profile which is managed by the network-manager (not just wireless ones).
But, there is no GUI for this type of advanced customizations. You have to use network-manager’s cli utility which is named as nmcli.
First of all, list configured profiles in console (just run these commands as normal user, sudo access not needed):
$ nmcli con
NAME UUID TYPE DEVICE
VodafoneNet-9A4BX1 0c56f145-05d0-4b72-946e-c9e4a95cb962 802-11-wireless --
miyaki 0bdcc6d1-133d-4fb4-81bd-137ebf6945f3 vpn --
TP-LINK_AP_18480 89d6dfff-1cc0-4833-8c8b-a2727be847d6 802-11-wireless wlan1
Wired connection 1 8cadcf80-2dd6-4198-a41b-f8e04380f1dc 802-3-ethernet --
...
Now we can get all of the details of a specific profile with nmcli con show command with giving profile name as last argument:
$ nmcli con show TP-LINK_AP_18480
connection.id: TP-LINK_AP_18480
connection.uuid: 89d6dfff-1cc0-4833-8c8b-a2727be847d6
802-11-wireless.ssid: TP-LINK_AP_18480
...
ipv4.ignore-auto-routes: no
ipv4.ignore-auto-dns: no
ipv4.dhcp-client-id: --
ipv4.dhcp-send-hostname: yes
ipv4.dhcp-hostname: imac
^^^^^^^^^^^^^^^^^^^ ^^^^
...
DHCP4.OPTION[1]: requested_routers = 1
DHCP4.OPTION[2]: requested_domain_search = 1
DHCP4.OPTION[3]: network_number = 192.168.2.0
...
It gives very long output, I just include some parts of it.
You need to change ipv4.dhcp-hostname option which has current value as imac according the above output. To change a variable, nmcli con modify command must be used as below:
$ nmcli con modify TP-LINK_AP_18480 ipv4.dhcp-hostname "my-custom-hostname-01"
After that, network-manager will use your newly configured hostname when connecting to this wireless ssid network.
You should also check nmcli man page for other usages: http://manpages.org/nmcli