If you are trying to spell the word so that you can use that spelling to convey the word - i.e., if you are trying to write dialog and one of your characters wants to say "us-[-...]" -- then I think it's important to try to echo the spelling/presentation of "usual." Otherwise it's going to be hard for readers to quickly understand that you're trying to convey a shortened, slang version of an extant word.
But if you are just trying to spell it for funzies, because after all how would you spell it, then I lean strongly towards a spelling that begins with a Y. I haven't settled on the rest of the spelling yet - I am thinking of rhymes, a la thenewgreen - but definitely, that Y gives you the right starting sound more succinctly and legibly than a multi-vowel mashup does.
Other partial rhymes besides luge: bourgeois (first syllable) , cooze, hues/huge (still only partial), lose, mews/muse, puget - as in sound (first syllable), rouge, stooge, yous/youse,
Although rouge and stooge sound the closest I'd think "youge" would look really confusing on paper...It doesn't tie back to "usual" clearly and out of context, I know I'd try to pronounce the g as a hard sound not a soft one. "Youghe?"
However you wanna spell it, it looks fucking terrible written down.