I wouldn't go as far as to guarantee it, you can't always predict how crazy a fish gets with breeding hormones, but from my experience I'd try it, could make an interesting tank. For me, rams have been feisty enough to stake out a defensible nook in a reasonably peaceful SA community, including some larger fish, like Mesonauta, or a big angelfish pair. They were bigger tanks, but also larger and more fish.
Of course, with a keyhole pair, you could end up with some fry to deal with. I've had breeding pairs, such as geos, in community tanks, which meant I could choose to save fry and siphon them out to raise in a growout tank at some point, or let nature take its course, in which case a few might hide well enough to make it anyway, which may or may not happen with a BN and plenty of tetras in the tank.
Have you ever had any experience with Laetacara curviceps? Seems like an interesting tank mate for the Keyholes.