As you noted, Qube does not have Combustion support "out of the box". However it does have a command-line jobtypes (Commandline and CommandRange) that allow for any commandline call to be executed. This has been successfully used for both custom studio applications as well as 3rd party applications.
As of Qube 5.2, the SimpleCmd interface expands upon the commandline available. It allows one with just a few lines of description to add a "Submit" menu item and submission dialog to the GUI. It has already been applied for XSI 6.0 and other applications like Cinema4d. Documentation for creating a SimpleCmd can be found under .../qube/api/python/qb/gui/README_SIMPLECMD.rtf.
If you can put together a commandline to render using Combustion, it can then be hooked into Qube's interface quite easily. Please let us know when you get something working or if you need further assistance.
