It's a very good investment to spent a week or a month (typically depending on age) to really learn hiragana. Learn hiragana and katakana in parallel. Use an app like 'Kana mind', for example, to enforce memorization.

Using romaji will, due to how the brain works, forever keep you out of learning kana properly, even when you see both at the same time. Learning kana is a small effort for a lot of gain.

Same, I love the idea - but I cannot read the hiragana/katakana (heck even: はい hai yes <- would work well).

Edit: Decided to make my own firefox addon to do it, no worry about daily limits and I can simply update a json file with more words when I feel like I'm remembering things.

I don't know a tool like this.

But this feature seems to be a fairly common requirement; we can consider add a switcher to implementing it.