Yes you can do renders without maya user interface but playblast is a user interface gimmick, it uses the onscreen drawing of your opengl context.l I think i was pretty clear on this, thats why i suggested hw render instead.
So you can do EVERYTHING* other except playblast and old style hw render without opening a maya graphical user interface (GUI), since those are the only things that rely on the gui to do output. However your still running MAYA, the inetrface is just one layer of the tool. This is not in contradiction with your other requirement, to do a playbalst i dont actually need to load 90% of maya just one very small part of maya.
Making a droplet out of ANY script is easy, gui or NO gui, tough i dont have OSX so i can not make a example for osx. But thing is this is very doable takes about 2-10 minutes to do. But again because hw render is allmost exactly the same thing and it can open without a GUI you should use that innstead.
PS: The real question is why are you having problems auto generating movie files out if sequences, since for any droplet mechanism its just one extra call to pack them up form your point no difference after that. However, for reasons of sanity you want to ALLWAYS use sequences of loss less images instead of a movie files, complaining about this extra step is a bit like complaining that you got change back form a cashier along with receipt for warranty. The step can be can be automated with typing 10 characters easily so you never need to do it yourself after that moment.
But why stop there? While yoru at it why even open the software you use to put them together in, you could make the droplet automatically make the movies AND open the editing application with the things put into yoru timeline as you need ready for you to tweak with 5 more minutes of work. In fact you could use a day or 2 of your life and you could probably instruct your editing application to automatically do this for you too and not need a droplet even, just dropping the maya file into our edit app could be made to trigger the playblast and place it in the app.
- yes i mean everything, except maya user interface stuff. I commonly use maya just trough a ssh text interface no magic in that. And yes i also frequently animate without the gui present.