If you can't select all the way down to 0 or 1 sat/b (as has gotten nextblock confirms when I looked at recent blocks within the last day), it's not 100% custom.
And they support this. They always had the option to allow you to manually customize the exactly amount you want to pay on fee with a textbox, and they also have a option to enable dynamic fees, where they will give you a few options based on calculations on their side - just like I said above.Tools -> Preferences -> check "Edit fees manually";
WTF is that a screenshot of? Not Mycelium Android...
Electrum.
Ah, whoops, thought it was Mycelium being discussed. Only ever have seen the Electrum GUI for OSX & Android. But in any case, I had to do this for Electrum: https://github.com/TheButterZone/electrum/commit/fca33e290015b475463481fc9b6626489aad5495