Hi! It's been a while, but I have some updates on this project. 
I got a working version of AAB support in MIT App Inventor 2.
During these past weeks, I worked in unifying both current export options and then researched the procedure to build an AAB. After some versions and commits, this branch, forked from MIT sources, is capable of building these Android App Bundles.
You can find some Pull Requests with changes of the project on that same fork.
You can go to this test server I just set-up to test by yourselves the AAB export feature. Just click in "Export", and the Android App Bundle choice:
https://aab.appinventor.barreiro.xyz
Disclaimer
It has a very cheap buildserver, so please don't force it too much. 
Android App Bundles work in a different way than normal APK. It basically changes how an MIT App Inventor project is distributed through Google Play Store. I have written this little developer document explaining how to work with Android App Bundles in MIT AI2:
I strongly encourage to read it so you can get more relevant info about AAB as an App Developer, specifically regarding signing both AAB and generated APKs with App Signing.
In addition, I will also make a tutorial for MIT Reference, so everybody can see with all details how to upload an AAB to Play Store using MIT App Inventor.
I would really like to know all your thoughts on this and get some input. If you encounter any issues when using it, some usability changes, questions about using AAB, possible concerns, etc.
Thanks!