I use Android Assistant for this (and a few other things too). In its battery saver settings, it has a screen timeout setting that still gives the option to never time out. Works fine, and even maintains brightness level when charging (the stock debug option still dims the screen after the timeout period if timeout active). The downside is if you go into the device settings menu and access the display settings for anything (change auto-rotate, brightness, or do nothing), timeout is re-activated until you go into AA to turn it off again. Seen this happen with other apps to stop timeout, so may not be able to bypass the reactivation. Adjusting brightness and such by other means, like the power settings widget doesn't reactivate the timeout.