I'm not sure whether this is the best public place to ask; the man who could best advise on this implicit subquestion is probably @Peter Constable
A sad feature of Tai Tham font development (though Apple may laugh) is that the best way of developing a Tai Tham font, though perhaps not for the faint of heart, is to avoid the Universal Shaping Engine (USE). With the HarfBuzz renderer, this can be done straightforwardly by having no script table in GSUB or GPOS for the script in question. The shaping and positioning are then done according to the features provided for the default script ('DFLT'). This also seems to work for CoreText.
I am not clear what the behaviour is intended to be on Windows 10. It seems to have evolved over the course of Windows 10, and I would like to know what the long term intention is and what the timescales are. In Notepad and Character Map, it seems that for a font with no GSUB or GPOS for the script in question, the USE will be run using features for the default script. For MS Word of Microsoft Office Standard 2016, it seems that the USE will be run with all features considered devoid of lookups, except that features for the DFLT script can be enabled in the normal fashion for MS Word. (There are some anomalies I have not yet worked out.) It seems the dotted circle will not be inserted if not present in the font. Will the MS Word behaviour change to that of Notepad? Or will there be a mechanism to opt out of the USE?
What happens with other renderers?