Any colon prefixed syntax can be made to work because it is enough for the lexer to detect and handle... so
:{defined varname}
may be an option, although I do not like the space much because it adds some fuzzyness in the lexer which has to process it. It is probably doable, though. I like having a "?" because there is a question. Other suggestions somehow in line with your proposal could be :{?varname} :{varname?} what do you think?
