Is it possible to alter the length of a single system on a page of several?

Consider the enclosed score. I would like to reduce the length of each "Alleluia" system to a more natural, less spaced out length. Ideally, I'd really like to centre each. I am imagining that is not a feature that I can do by some kind of selection and then fiddling with the inspector. So I tried adding a horizontal box to the front of a system (select bar, add->frame->insert H frame), but selecting a bar then add->frame->append H frame simply tacks it onto the end of the score, rather than the selected bar. So that's no use.

What I'd also like to do is be able to control the filling of a section and not just a score. Then at least I could achieve centring that way. It would also (IMO) look nicer at natural ends of sections, at least for this style of music (e.g at the end of 2 ABC). It also makes it more obvious, over and above the extra inter-section spacing, which systems belong to which group - without an ugly // mark in between.