I have now included Tamil into ‘Munson’.
Most of it works quite well. However I then tried to incorporate the pre-reform characters into the script as well. I first tried adding them as Stylistic Alternate1, but when used in a word processor or a DTP program they didn’t come out, the characters stayed as their base forms. So I added them as Stylistic Set1, Historical Forms1 and Contextual Alternates1, none of them work. They work in Font Creator but in Libre Office, Ability Office, Inkscape and Scribus the characters remain in their base forms when the open type feature is applied to them.
What am I doing wrong and how can I fix it ? 
Munson-Regular.ttf (371 KB)
1_Munson_Roman.fcp (300 KB)
I don’t see why it would fail. Since this seems like a specialized issue, it might be best to ask at typedrawers.
I am no longer on TypeDrawers.
I don’t know why you are no longer on TypeDrawers, but that is the place where experienced typographers meet and provide help.
I know for example John Hudson does provide assistance and he happens to be one of the persons who made Nirmala which comes with Windows and has support for Tamil.
Because those ‘experienced typographers’ (i.e. the professionals) don’t like giving help and support to those typographers who make free fonts. They are elitists.
If you are an newcomer to font design and you ask a question then you will get plenty of helpful advice, but most of these questions are simple to answer.
If you are a professional and ask a complex question then you will get plenty of helpful advice because the professionals help each other, after all they may need help in the future.
If you are an amateur (i.e. a producer of free fonts) and ask a complex question then you will get very little help if you get any at all. The attitude seems to be “why should I help someone who is producing a free alternative to my paid font ?”.
I left because of the elitism which had been displayed for some time but it came to a head because of one particularly toxic thread about the proposal to charge money for font critiques.
I am sorry about this; I still hope someone can provide valuable feedback.
I was able to use the font in Word. Just be sure you provide the font name in both Latin text and Complex scripts fields.
கீ க ீ கீ
Hope this helps.