I see, and yes, I can reproduce this. It's not a generic limitation of the kernel, but rather a limitation of the current way that network interface configuration is stored in the Config Registry. See this bug for more details. The Univention Management Console is simply preventing you from running an error there due to that limitation.
You could implement a workaround by modifying how the script
/etc/network/interfaces is generated. Fortunately the template that generates that file supports adding arbitrary options at the end of the
iface block by setting appropriate UCR variables. For example, in a bridge configuration you already have two of those:
interfaces/br0/options/0: bridge_fd 0
interfaces/br0/options/1: bridge_ports eth0
You could add another one that uses the
up keyword to execute the command that adds the IP address, e.g.:
ucr set interfaces/br0/options/2='up /sbin/ip address add 192.168.100.254/24 dev br0'
Attention: the act of setting this variable not only triggers rebuilding the generated file
/etc/network/interfaces but also an immediate restart of the networking.
Note that it's quite possible that such a manual modification will be lost as soon as you re-configure the interface via the Management Console. I advice that you actually give this a try: set the UCR variable, verify the address is present, go to the UMC, modify the interface somehow, and observe how that affects both the UCR variable and the generated