There's actually Unicode and CSS support for it, but it just isn't widely integrated / used.

https://ja.wikipedia.org/wiki/%E5%9C%8F%E7%82%B9

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.