The thing about hentaigana is that it's more or less an open set. I don't think there is the standard set of hentaigana that scholars agree upon. At a quick glance, juki-gana does not seem to be identical to Unicode's hentaigana set (https://www.unicode.org/charts/PDF/U1B000.pdf), although I'm sure the intersection is non-empty.
So all they need to do is use a font that implements it.
All hentaigana are available in Unicode 10. They can be found in the Unicode block 1B000–1B12F (Kana Supplement + Kana Extended-A).
For example, U+1B004 𛀄 HENTAIGANA LETTER A-3
It is completely brutal.
I reviewed this one 384 times already: https://www.benricho.org/kana/kana-img/f4f6.gif
The lapses are just crazy.
My second most reviewed, at 374 times, is https://www.benricho.org/kana/kana-img/f465.gif
Third, at 306 times, https://www.benricho.org/kana/kana-img/f4b7.gif
It's also serious because names are accepted as signature using hanko (official stamps) for official business and procedures.
By the way, there’s no requirement that your hanko matches your name and they only need to be registered for large purchases, like a house or for business use. They are however kept on record by your bank and sometimes compared digitally. By law foreigners can always use a signature instead of hanko but it’s not recommended as the comparison will often fail. On the other hand, online banking ans ATMs are good enough now that it’s been a good five years since I set foot in a bank.
There is a requirement for it to match your name (or registered alias) if you want to register it as a 実印, no?