What do you say about development client Telegram? There is open source code and good APIs. I wanted to write a client application in Java, but it would be better if the app will write in the native platform language.

View info about Telegram API: https://core.telegram.org/api#getting-started
Source code Qt-based clients (Win, Lin, X): https://github.com/telegramdesktop/tdesktop
Source code Android client: https://github.com/DrKLO/Telegram

And unofficial CLI-version client: https://github.com/vysheng/tg

I think if You post the source code, then other developers of applications for Maemo will join You.

Thx!