Presumably you are already aware that you can create a .qtz file that can act as a screensaver, without a .saver extension from within Quartz Composer.
What about wrapping your .qtz screensaver composition into a QCView and wrapping that into a ScreenSaver?
You would need to include the Quartz Framework in your project, just starting from the xCode Project Template window.
In fact most QC based screensavers stick with being just .qtz products and eschew any form of additional GUI apart from their published up ports as preferences.
If your composition depends upon Leopard specific QC patches or Snow specific patches then you won't be able to target earlier versions of OS X.
Long and short of it, until or unless .qtz's cease being a supported file format, they can be made into a ScreenSaver via the Composition Repository.
Presumably you are already aware that you can create a .qtz file that can act as a screensaver, without a .saver extension from within Quartz Composer.
What about wrapping your .qtz screensaver composition into a QCView and wrapping that into a ScreenSaver?
You would need to include the Quartz Framework in your project, just starting from the xCode Project Template window.
There's a really great ScreenSaver project tutorial here http://cocoadevcentral.com/articles/000088.php and here http://cocoadevcentral.com/articles/000089.php with example project files, though they aren't focussed upon QC.
You will also need to refer to https://developer.apple.com/library/mac/#qa/qa1666/_index.html for avoiding any Snow Leopard or higher issues with this project. That said the venerable http://www.jwz.org/xscreensaver/download.html source code, once updated, will output a working ScreenSaver and has over 50 other ScreenSavers to go.
In fact most QC based screensavers stick with being just .qtz products and eschew any form of additional GUI apart from their published up ports as preferences.
If your composition depends upon Leopard specific QC patches or Snow specific patches then you won't be able to target earlier versions of OS X.
Long and short of it, until or unless .qtz's cease being a supported file format, they can be made into a ScreenSaver via the Composition Repository.