I've released a small open source plugin for Geany, which calls Prettify on the edited file.

https://github.com/senselogic/GEANY_PRETTIFY

It's mainly targeted for programmers like me who use Geany to develop in D, but it works also for other languages (C, C++, PHP, HTML, CSS, JS, etc).

In the background, it simply creates a temporary file with the same file name and extension in /tmp/, calls Prettify on it, and then loads the prettified file.