Is there anything useful, besides executing and ransoming, that I can do with an imprisoned liege?

Leave him imprisoned. He's far more useful to you that way.

Keeping him imprisoned will mean the HRE will be a regency. Regencies have a lot of restrictions on what they can do, which is perfect if you want to become more powerful.

Regencies are also vulnerable to factions. Regents typically fold to faction demands more easily and will automatically accept any faction if they are in it. Either become the regent or get him involved in your crown authority faction and you'll be able to lower it all you want.

Having the enemy leader imprisoned also means you automatically win any wars against them. If you want a quick and dirty method to drop crown authority, start a faction and press it. You are now at war with your liege, with 100% warscore, so you can enforce demands right away. This will release him though so it'll only work once.

Of course, this isn't limited to Lower Crown Authority wars. If you were to make a faction about something else, for example, making someone else emperor, that'd work too. And since anyone eligible to vote is eligible to take the title this way...