Honestly, I think the tag is best limited to parodies. Something like hestia_(danmachi) ~blue_ribbon ~arm_ribbon should catch the canon costume examples. And parodies can involve strings that are not necessarily ribbons such as post #1976505 and post #1976380, so I don't think we should do this implication anyway.
It's probably far more of a headache to try and keep Hestia images out of a tag that's inspired by her than it is to make people search rei_no_himo -hestia_(danmachi) when looking for parodies.
Agreed in regards to not doing the implication, though.