It's likely you're going to have to get your hands dirty if you want this to be better than what it is.
It looks to me like the next steps are creating an application bundle (i.e. a .app) from the built application. Some googling leads me to https://wiki.gnome.org/Projects/GTK%2B/OSX/Bundling
The best solution would be to convince one of the Homebank developers to use this to setup an automated build that spits a binary somewhere that you can then reference in a cask. I wouldn't anticipate a high chance of success given the main developers are linux focused.
A second alternative is to do this yourself on your local system.
Once you've done this once, you could script that change and provide an update to the makefiles for the original developer, or alternatively to the homebrew formula.