There isn't anything built-in like Lapis's database functionality. However, it should be possible to use existing database libraries. For instance, you should be able to use something like lua-rds-parser if you're using OpenResty. However, I'm not sure what specific OpenResty incompatibilities there are with trying to use, for example, LuaDBI. If you use Xavante, I would think these libraries should work fine. Another possibility might be to *borrow* Lapis's database code (while trying to rip out the Lapis specific pieces, like lapis.config.) This may or may not be a big task.
In any case, this is likely my next area to focus on. At the very least, figuring out exactly what is and isn't going to work to document within the README. I doubt there will every be anything ORM-like directly supported. It could be a separate project, because you likely just need to setup the connection and map the table to be passed to the requests.