The last post on this niche subject, I hope.
By dint of much trial and error (and with the inestimable help of Iyanaga-san), I have discovered that Word (for Mac OS) does in fact support the Noto Sans Siddham font. I was misled by the fact that when you install it, Word refuses to show it in the
font list, so I naturally assumed it had been rejected. However, if you copy syllables created in Noto Sans Siddham from the Mandara-ji site and then paste them into your text not only are they accepted but at that point the font shows up in the font list.
You can now print your own dhāraṇī!
What a serif version of this baroque font might look like is difficult to imagine.
Richard Bowring