I’m not convinced that this will be possible. The way I understand it, the root problem is that 240p and 480i have slightly different framerates, and the regular OSSC has no capability to adjust its output framerate. Even if the OSSC were to transform 240p to 480i, the resulting framerate would still be different from native 480i from the same console, and the OSSC wouldn’t be able to do anything about that; the 240p<->480i switch would result in a regular mode change from the OSSC, which in turn would mean an HDMI blackout. We already see this with line-doubling 240p and 480i to 480p–the frame sizes are the same, but the framerates are still different, and you still get a blackout.
Where those cheap scalers differ is that they use framerate conversion and scaling so they’re continually outputting at a consistent resolution and framerate, regardless of input (misinterpreting 240p as 480i is a separate issue); so, if the scaler is configured to always output 480p@60Hz, it will maintain that output, even if you switch from, say, firstname.lastname@example.orgHz to 576i@50Hz to 480p@60Hz.
 I think a line2x (laced) mode for 240p/288p might be reasonable for getting around 240p incompatibilities, like when running OSSC output into a cheap scaler to smooth-over framerate issues.
 Notice how HDMI-native consoles, like the Xbox 360, PS3, and newer, don’t continually switch video modes during games; they pick one video mode, on startup and/or when starting a game, and then stick with it.