Consider the .svg page with the following markup:
[[
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
<g writing-mode="vertical-lr">
    <text id="text" font-size="50" x="50" y="50" style="-webkit-user-modify: read-write">Hello</text>
</g>
</svg>
]]

Open the page in MobileSafari. Tap the word "hello". Then nothing will happen, but I expected the keyboard to appear and a caret to show up and I should be able to edit the text.

Note that if you add an explicit taborder to the <text> then you can tap on it to start an input session:

[[
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
<g writing-mode="vertical-lr">
    <text id="text" font-size="50" x="50" y="50" style="-webkit-user-modify: read-write" taborder="0">Hello</text>
</g>
</svg>
]]