Hi Katie -
As far as I understand, the difficulty is that Kayan uses a combination grave and acute, aligned next to one another, as a diacritic. We’re hoping to be able to include a unique feature to support that combination in the next version of our roman fonts (including Andika) that is due out early next year. Andika would then support the single-story ‘a’ you want.
You should be aware that combining the two diacritics in this way is a very unfortunate choice for the orthography. I can pretty much guarantee you that our fonts are the only fonts that will ever support that side-by-side alignment. That means that the language community will never be able to see that combination aligned in that way on any standard OS fonts, never be able to text one another on mobiles, never be able to create web pages that contain their language that are correctly rendered on all platforms. The long-term cost to the language community is very high and painful, and is likely to contribute significantly to long-term language loss.
We will do what we can to support their orthography choice, but the big computer companies (Apple, Google, Microsoft, Samsung) will never support the side-by-side alignment. I’d be happy to discuss this long term problem - and alternative options - with you further if you would like.