It has admittedly been a long time since I've played EFU, partially due to some recent lag issues but even when I was active I was never quite certain on the server policy on fake religions and false deities. While of course you couldn't field a cleric of such a "god" I would imagine the cultist sorcerer background would make for quite a nice fit. So my question is essentially can I fill my deity field with the name of a made up god. I am asking this because I've really wanted to start up some kind of crazy cult group concept. If it is frowned upon I completely understand, I simply want to know so that I don't commit any taboos/rule violations.
There is absolutely no reason to frown upon a quirk, lying characters or absolutely crazy beliefs. Whatever characters do for their own reasons, can be pretty much anything and I don't see why we'd ever frown upon it. (Mind, everyone can think of some instances where some good use of common sense is wise, since we never are so much in character that we wouldn't have some manner of OOC agenda).
You can do whatever you want, but there can always be consequence of it. And one of such is that you likely don't get any kind divine powers as a cleric.
Relic Guardian
As RPG said, relic guardian is a great choice for this. They drawn their divine powers from powerful artifacts rather than a specific god. Alternatively, you could play a regular cleric that has a deity in their field but is a heretic of the faith or has an unorthodox set of beliefs.
Something I remember The Team saying a long long time ago is that they'd much rather people put any arbitrary name into their deity field, than to leave it blank. I believe that the worship of Frederick Bresley mostly started as a joke in IRC because somebody had put that name in their deity field in EFU1.
So yeah, by all means, you should feel encouraged to put whatever name in your deity field that best represents your character's values, regardless of whether it's a being that grants powers, or even one that exists in the setting.
Remember, the line between being crazy and being right is a fine line indeed - maybe that made up god actually does exist, or maybe something else will end up listening to your prayers. If you're awesome enough it might even be likely.
Kotenku nailed it.
I am sure that many people over the year have put Tau in their deity field. Mostly think of it as writing down what they revere.