After fixing tdf#133661, the documents created using Windows Explorer's ShellNew command follow some LibreOffice settings (language, fonts). However, they still do not honor user-defined default templates (as configured in File->Templates->Manage Templates). This means that the two methods of creating new LibreOffice documents (from within LibreOffice application, and from Explorer's context menu), which from user's PoV should be interchangeable and give identical results, in fact behave differently: the former method does what user told to do, while the latter ignored user default template setting.

With the fix to tdf#123476, LibreOffice now opens 0-byte files with ODT/ODS/etc extensions as empty documents in the corresponding component. This paves a way to implement the missing functionality: process such files to make them use default templates, and then create empty files in Windows ShellNew handler (i.e., drop soffice.* files in C:\Program Files\LibreOffice\share\template\shellnew, and replace "FileName" with "NullFile" in ShellNew registry subkeys [1]).

We should detect that the opened file that uses our native filter is 0-byte file (tools::isEmptyFileUrl) somewhere before the call to SfxObjectShell::DoLoad, and use SfxObjectFactory::GetStandardTemplate to get the default template for the component service. Then we must use the template to initialize the document, but unlike the normal creation of a new document from a template, we must make it have the name and location of the 0-byte file.

I don't have a specific proposal where should this happen, and how to implement the substitution of temporary storage (that is used for newly created files) with the 0-byte file, thus not marking this an easy hack.

[1] https://docs.microsoft.com/en-us/windows/win32/shell/context#extending-the-new-submenu