This is an obscure use case, so I'm not surprised that there's nothing about this online. I already know how to type accent marks in general, and how to change keyboards. However, I was not able to find any way to type e.g. m̃ using the Lithuanian or ABC Extended keyboards. I thought option-N followed by M would work in ABC Extended, for instance, but it doesn't. Here is a complete list of the characters I would like to be able to type:
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.693.9500&rep=rep1&type=pdf
Note that only a subset of these pose an issue. For example, I can obviously type é, ȩ, and č in either of the keyboard layouts above.
Any advice is appreciated. I would prefer to not have to pick characters from the symbol selection table.
Best Answer
For info on how to add combining diacritics to any character with ABC Extended, see this page
http://sites.psu.edu/symbolcodes/mac/codemacext/
For example, m̃ is made by typing m and then option-shift n. ė̃ is made by typing option w, e, then option-shift n.
The keystrokes for adding acute ́ and grave ̀ are option-shift e and option-shift `.
You might want to create a customized version of the Apple Lithuanian keyboard using Ukelele to adding keys for the combining diacritics used to indicate stress. Here is one you can try.