First Idea:
Add button select the type of the model: "ModelBiped" and "ModelBase".
Second Idea:
Should be a button (only for Minecraft) "Choose the Model Type". If the selected Biped will be created as character in the screenshot. If Base is empty.

Armor Example:

P.s. If not difficult, then it is advisable if you export to remove the abstract model.
Third idea:
Since Java exporter models or who have not tested, suggest to the new release, test build. To recruit testers and to give them the test builds.