I'm an inexperienced Japanese user and know only about 30 kanji or so. So every time I ask a question on Japanese SE I specify "Please use kana or furigana only, no kanji." It seems simpler to have the tag kana-only or no-kanji or maybe also kana-and-furigana-only, to specify questions which only admit answers using hiragana, katakana or kanji with furigana. This would personally save me the trouble of writing the same sentence for every question, and may be useful for other newbie Japanese learners who get confused or off-put by kanji-heavy answers.
To get around the problem, I suggest that you state your level of learning very clearly in your question somewhere. I'm sure people will gladly accommodate you and use easier/simpler words and expressions as well as add furigana to the kanji, or just avoid kanji at all. And if someone doesn't, others can always edit it in if they notice.
I wouldn't support this proposal. Tags are meant to describe the topic or contents of the question (e.g. grammar means that a question is about Japanese grammar).
kana-only tag the way you'd like it to be used would be like a meta-tag. It wouldn't describe the question's topic, it would describe the script you would like to be used in the answer. This would be confusing as it would mean the usage of this tag would be different to all other tags.
Understood the way all other tags work, the tag kana-only might be taken to mean "the question is about words/phrases/passages written in kana only".