As far as I know, there isn’t a native way in Notepad++ to change the tab setting on a per-UDL basis.
That would be nice, and in theory, you could put in a feature request. Unfortunately, the UDL system doesn’t often get bug fixes, let alone feature requests.
However, there may be a workaround; the underlying Scintilla library has a SCI_SETTABWIDTH message, which you can access through a variety of means, including the PythonScript plugin (editor.setTabWidth(2) will change it to two spaces per tab). With some logic, you could write a PythonScript that determines whether the active editor is editing a BiBTeX-UDL-based file, and if so, run that PythonScript setting. If you have PythonScript, or are willing to install it, someone here should be able to give you pointers on how to implement that logic.
<edit> You might also need to change SCI_SETTABINDENTS, depending on your Settings > Preferences > Language > Tab Settings > [Default] settings </edit>