Assuming you are specifically talking about their apphosting services (aka not dedicated servers), then the answer is mostly no. I say mostly, because some services you can manage to install and configure without needing root access or anything else that’s blocked on HBD appboxes.

As for your example of AudioBookShelf, I’m pretty sure it wouldn’t be possible, although maybe you can download the source from github and mess around with it if you are so inclined. I haven’t tried and I don’t have experience with ABS either, but I assume it’s not a possibility.

I was an early-ish adopter, and I loved the project, but have reluctantly kept using the app since they dropped basic sms bridge support on android.

Being sold to Automattic is a bit of a concerning thing for me tbh, and some other design choices made since the sale have frustrated me as well, but if they reimplement the sms bridge for android, it could go a long way to keep me on board, as I don’t particularly feel like selfhosting matrix… Will have to wait and see how things progress.