Is it possible to load DLLs and TX files from a different folder than where the main application is installed, e.g., a subfolder of my installation such as ODA\DLLs?

For DLLs and .tx modules you link with, it is the same as for any other DLLs. Their folders should be in the PATH variable.

For .tx modules that are not linked (loaded dynamically with loadModule()), you can override OdRxSystemServices::findModule() and return the full path.