I can't find a way to do this, but it would save me a huge amount of time in so many situations.

Are you working with packed geo? If not look into it…

I am, but how does that help?

I'll often be loading Alembic files with complex hierachies. Selecting the objects I want in the viewport would be trivial, since the organization makes sense in 3D space, but Object Merge from selection doesn't seem to exist. Picking them from the floating operator chooser or using paths and wildcards is a pain.

you select the packed objects in the viewport and create a group
the object merge sop also lets you choose a group

You can try the ‘Extract’ tool from the scene view. Select the primitives you want to merge into another object, and hit tab->Extract. The tool creates a new object containing an object merge, instead of letting you pick an existing object to merge into, but at least you can cut and paste the object merge where you want it.

@Olaf: Thanks, that would work, but I'd have to merge everything into a single object beforehand.

@jsmack: That's great! It creates one Object Merge node per object instead of a single one, but I can live with that. Thanks!

Edit: @Olaf: …although it would work with bundles… Haven't used them much but it would be perfect for this, I guess.