Is there any way to substitute a custom New... program on the File menu of Open Roads Designer?
there are two issues in your question:
How to customize backstage (or QAT): As David wrote, the customization cannot be done by a user, because such tool is not available in MicroStation. But technically it's possible using XML (or rsc) configuration file that is described in MicroStation SDK documentation. Unfortunately explanation of backstage is missing, on the other hand because it's structured similarly to the rest of ribbon items definitions, so I guess it would be possible after some time invested into testing and playing with the file.
How to block / replace an item existing already in backstage (or ribbon): In my opinion it's even more complicated than how to customize something. The system how the whole ribbon (incl. backstage) is defined is pretty powerfull and complex, so it allows even to replace already existing definition by a new one (it's part of definitions conflic resolution mechanism). Mandatory part of the replacement definition is to know definition name of the existing definitions, but such list does not exist.
My conclusion is that it's good question, that probably can be done somehow from technical perspective, but not by user.
And in fact it's not the best solution available I guess, because when you will replace all New items in GUI, there will be "create drawing" key-in still available. Maybe (but it's also technically complicated) native code (MDL) application would monitor commands input queue and to block all detected "new file" commands. But it also cannot be done by user.
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