I think I've found a solution that works for both, the suggested feature of having a list of predefined composers, and the existing behavior of listing all the different values that are available in the select files.
The new version Mp3tag v3.17b allows for adding custom list values when configuring a field via Options → Tag Panel. These values are then available in the drop-down list of the respective field on the Tag Panel.
If the option Prefer values from tags in list at Tag Panel is enabled (it's by default), the previous behavior of only listing the values from selected files is used. If the selected files have no value stored for the field in question, the custom values are also available to choose from.
Really hope that this is a solution which is beneficial for everyone 