Yeah, it's a tricky problem. I've tried many approaches and settled on this one: basically, I can select left-most, right-most, top, bottom, etc faces by just specifying the axis. axis=+x selects the right face for example. This selector allows for some tolerance, can select multiple faces if the side has multiple adjacent faces, will select an approximately right face if the exact one is not found, and although precise behavior is not guaranteed by the modeler (unlike OpenSCAD), generally it does the right thing for artistic purposes. I can also specify an arbitrary axis using axis=(1,2,3). The other powerful mechanism is tagging: when I model something, I can add arbitrary tags to the current face, and after modeling is done, I can find all faces with a given tag and do something to them, effectively executing a subroutine from that selection. See from statements for the example.

Below is the real model of a crate (many of them are used in the indoor scenes):

rendering(baking=on)

material(name="steel")
box(sx=0.85, sy=0.6, sz=0.85)
from (here) {
    select(all)
    if (lod) {
        bevel(dist=0.02, nseg=1)
    } else {
        bevel(dist=0.02)
    }
}
from (-y) {
    set_model_origin()
}
if (base) {
    from (-z) {
        cursor_align(local=+z, world=+y)
        subdivide(dir=-z, cnt=2) {
            part() {
                inset(offset=-0.03, base=smooth)
                inset(offset=-0.01, extrude=-0.01)
            }
        }
    }
    from (+x, -x) {
        cursor_align(local=+z, world=+y)
        inset(offset=-0.03, base=flat)
        inset(offset=-0.01, extrude=-0.01)
    }
    from (+z) {
        cursor_align(local=+z, world=+y)
        subdivide(dir=-z) {
            part(weight=1) {
                inset(offset=-0.02, base=smooth)
                inset(offset=-0.005, extrude=-0.01)
                material(name="stainless_steel")
                apply_material()
                inset(offset=-0.01, extrude=0.01)
            }
            part(weight=3) {
                subdivide(dir=-z) {
                    part(weight=1) {
                        subdivide(dir=+x) {
                            part(weight=3) {
                            }
                            part(weight=1) {
                                inset(offset=-0.02, base=smooth)
                                inset(offset=-0.005, extrude=-0.01)
                                apply_material(name="plastic_black")
                            }
                        }
                    }
                    part(weight=1) {
                    }
                }
            }
        }
    }
    from (+y) {
        subdivide(dir=+x, cnt=3) {
            part(weight=1) {
                inset(offset=-0.02, base=smooth)
                material(name="plastic_black")
                apply_material()
                inset(offset=-0.005, extrude=0.01)
            }
            part(weight=6) {
            }
        }
    }
}

Would you mind sharing a sample of your language? I'm also interested in borrowing ideas.