I'd like to be able to keep track of which static IP addresses are assigned to which physical devices, but unfortunately the "Active Host Name" field in WinBox is not populated for a lot of the devices. Is there any way to change the "Active Host Name" field that corresponds to certain MAC addresses? Or is there any other way to assign labels to MAC address or IP addresses? I can't seem to find any way to do this within WinBox or the Web UI.

That is based on what the client sends back to the DHCP server, you have no real control on it on the server side - if you have control over the devices, you should see about configuring them correctly to give the hostname in response. Most I see in general do but some definitely don't.

What you can do though if you want to track specific MACs is make them static and add a comment to the static lease with the hostname/device description.

If I'm following your request, it's easy. First let me make sure that I understand what you want. On your DHCP server, you have multiple devices that have static reservations. Because many devices don't show an active host field, you can't tell them apart. If that is correct, here is your solution.
On the Leases tab on the DHCP server, the active host name comes from the device. You have no control of it. However, it you add a Comment, that will give any comment that you wish on line above the information for that device. There are two ways (within WinBox) to assign a comment. The first is to select the device for editing and click the "Comment" box, it will bring up a box where you can enter a comment. The other way to enter a comment is to select the device (not double click it as if to edit it), and press [CTRL] M. That will bring up the same comment box.

Thanks for your help. I was hoping I could adjust the device names like I'm able to in Ubiquiti Unifi software, for example. From what you and the other response informed me, the best I could do is add a comment if I'm unable to get the devices to properly present their hostnames.

That is based on what the client sends back to the DHCP server, you have no real control on it on the server side - if you have control over the devices, you should see about configuring them correctly to give the hostname in response. Most I see in general do but some definitely don't.

What you can do though if you want to track specific MACs is make them static and add a comment to the static lease with the hostname/device description.

... There are two ways (within WinBox) to assign a comment....

It works if and ONLY if DHCP address is set to static. You cann't assign comments that way to any dynamic data in ROS