Using a HOB with a skimmer piece as a protein skimmer?

This would work, although not to the super high standard that a dedicated protein skimmer works (since they mix water around a filter to get it out of the water itself vs. just the surface of the water). Just make sure that the protein skimmer attachment isn't able to be knocked down easily, and that it goes through the filter vs. just being sucked into the output directly (I've seen some filters do this).

I think it might work for smaller tanks, like nano tanks if you can get a small HOB designed for them - and but for bigger tanks, it might be problematic since they don’t have a huge flow rate compared to canister filters, or use sums, etc.