This is nice, Kathy.
I must admit, and this is probably only due to a lifetime of assimilating Anglican liturgical language, that 'have mercy on us', whether here or in the mass, always sounds abrupt and metrically stumbly in place of 'have mercy upon us'. It's almost as though people go out of their way to offer a locution that has less grace. I'm sure, though, that those not conditioned by Anglican usage will probably disagree.

It seems to me that there is a degree of aesthetic and linguistic pleasure (not to mention, musical grace) in -
Hear us, O Lord, have mer - cy up - on us
...c.....e....g.....g,.......g.......c......b....g......a...g.

that is lacking in -
Hear us, O Lord, have mer - cy on us.
....c.....e....g.....g,.......c........b.......g.....a.....g.

Still, your translation is quite nice. I like it.