I know this is a bit old so your 'con may have already passed, but just thought I'd add this idea. At a con I went to last year there was a Seventh Doctor costumer who had actually painted the sweater vest- and it looked amazing! They found a fairly tight knit very fine yarn yellow sweater, made a question mark stencil, and carefully applied it. Nobody would believe it was just paint until they looked really closely.
Took a lot of slow and careful work, but not as much as learning intarsia knitting.