There's actually Unicode and CSS support for it, but it just isn't widely integrated / used.
Writing horizontally the dot would be above, so I'd assume there's support by composition.
It would still be a problem of IME and font support.
On input, there's already so many shortcuts and hacks (e.g. SHIFT/CAPS is already taken to force switch from hiragana to katakana) that it's hard to imagine some natural combination that could be memorized.
On the font side, there's already the battle raging for having proper jp fonts in smarphones instead of the chinese ones, so additional support for marginal features is an uphill battle to say the least.
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They accept suggestions, and people suggest emoji.
So suggest something that isn't emoji and it'll likely be accepted and added to the next version.
That doesn't help
Suggest the characters that you feel are missing... Was I not clear?
Hmm, it looks like the discussion about emoji has been removed/moderated from the page?
Anyway, my point was only that if someone is unhappy that emojis are being added, then proposing new non-emojis doesn't really solve their problem.
Hey Unicode Consortium, where you at?