That is correct - neither the RTF export nor the (standard) PrintDraft options in Paratext support ruby text. HOWEVER, as of a week or so ago, the relatively new standalone tool called PTXprint supports ruby text. There was an announcement made last week about its availability. Make sure you download and install the latest version (1.5) from the SIL site, and see how you go. There is no separate documentation for PTXprint, but each option has helpful tooltips (as seen in black below).
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If you have any trouble making it work, feel free to reach out through the PTXprint community page, or if in a sensitive project, email ptxprint[underscore][Email Removed] directly.