I'm also going to tag you, @Netduma Fraser, because this information may help clarify what I said in my previous post. However, I'm going to assume you and the rest of the Duma Team know how DHCP lease times work.
The DHCP Lease Time won't actually hurt devices on you network if they get dropped, assuming you don't have fine tuned settings specifically for those devices based on their IP. If you did have rules based upon the device's IP this is where you would whitelist that device for Duma's sake.
To be honest the "whitelist" feature isn't necessary aside from having an encompassing device tree. Once your device comes back on the network it would show back up in the device tree/table. If you wanted the devices to always be there then the "whitelist" should simply be the use of reserved IP addresses.
Let's take your smart tv scenario for example:
I'm your example you mention the smart TV being offline for X-amount of time and risking being "dropped" from your network. What the DHCP Lease Time is doing would essentially just release the ip address that was assigned to that TV and would remove the TV from the Devices portal.
The next time that TV is turned on and attempts to connect to the network it will still connect to the network. However, instead of it having the same IP address it did previously, let's say for example... A week ago, it's simply assigned a new IP because a different active device on the network grabbed the TV's old internal IP.
Just because a device is dropped from the DHCP Lease Time rules does not prevent the device from accessing network the next time it comes online.
At work I manage Cisco Meraki security appliances, switches, and AP's. I have the network stack configured to drop inactive devices from the allocation pool of they have not been active on the network for 24 hours. Anything that needs to have the same IP, regardless if the device is offline for longer than 24 hours, has a reserved IP.