Probably one of the best strategies would be to auto-generate MPL XML files from your database (somehow, this will most likely require some custom coding or scripting on your end). The MPL files will have to point to some kind of filename, so you will need to choose some kind of physical "file" to serve as the "host" in MC's Database for this tag information.
So, you'll need to decide how "atomic" you want to be with how MC stores the data. Do you want the "atom" to be the album (disc on the shelf) or the "track", in other words? You could do either. If the "atom" is the disc, then each "asset" could just have a field (say, a list-type [Track Listing] field) with the track listing entered all in that field (nicely formatted to be readable on-screen from Theater View, for example). Or, you might want individual "entries" in MC for each track on each disc (like a regular digital library), where individual tracks can be worked with, and mixed together maybe. In that case, then the track would need to be the "atom".
Then you'll need essentially a dummy file for each of these atoms (of whichever type) on disc somewhere, so you can import them into the library. You're not going to really ever want to "play" these assets, but you need to have a unique asset on the filesystem for each "atom" (whether by track or by disc above) in the Library. I don't think there's any reason your MPL imports can't each point to a single asset for each album, even if you want to have discreet track items that can be viewed normally in MC (though I'm not sure). I pretty sure you can't easily, and probably shouldn't, point multiple different MPL files at a single [Filename] because then Auto-Import will ignore future entries. Someone else can comment here.
So, what you'd do in your database extractor tool is have a "master" copy of some dummy file. Something small that you can import into MC. It might be nice to have these "dummy" files be actual audio files (you could record yourself explaining what to do or something and save it as a small MP3), because then MC will recognize them as "audio" and automatically include them in the right Media Type.
Then, the database tool would copy one of these files, along with an associated MPL XML sidecar file, into a pre-determined folder structure, where MC's Auto-Import can watch for them. In Auto Import you can set additional Tag On Import rules to do things that apply to all the files coming out of the tool (adding a tag indicating it is of Type "Vinyl" for example), which will be easier than hard coding it into your extractor tool.
That would let you do it all in one go, and you could map any data out of your database into the XML. It would also allow a continual process (first, so you can test little by little, and later if you happen to want to keep using your existing database to create new entries in MC). Plus, there are a variety of XML-producing toolsets available for pretty much any modern database or programming language you can think of, so it should be (if you're clever) reasonably simple to manipulate your data into the required XML and write it to disk, along with a nicely named duplicate of the master dummy file, into the appropriate watched directory.