Thanks for the contribution. This looks fundamentally reasonable. I'm still thinking about proper naming.

Note: We unfortunately have analignment parameter that only covers the horizontal alignment of title and content and is thus too generic and semantically overlapping the alignment here. But for backward-compatibility we cannot change that.

The new naming needs to be more specific, butitemboxalign feels very technical and it may be hard to guess what it is doing. Maybehandlealignment? If one wanted to be very precisehandle_vertical_alignment, but that's quite long. Suggestions welcome.

I'll also note that in Legend documentation, the term "entry" is also used for the marker+label combination:

alignment{'center', 'left', 'right'}, default: 'center'
The alignment of the legend title and the box ofentries. The entries are aligned as a single block, so that markers always lined up.

So, perhaps alsoentryalignment orentry_vertical_alignment - but I would argue that there's not much sense in talking about horizontal alignment of individual entries, so I slightly prefer the shorterentryalignment orhandlealignment.