Battletech

Probably not. It's abysmally slow so it's limits to tight cities unless you want decoys to draw fire. AC20 is a trap of risk/reward and it's ONLY good in tight urban environments, which effectively neuters most light mechs and a lot of range anyway, so having a gun that one shots most things as it's warning shot is a waste of points.

AC10 suffers from the same scenario as the 20 but weaker punch.

Maybe a custom build with BAP or ECM and LRMs? Indirect fire would be a great use and having lots of generally accurate LRM10s or 15s landing each turn is hard to endure. But light mechs with machine guns would be a threat so the urbies would need support and that removes them as rear guard.

Can the assault mech kill most things and the rear guard is there to deflect sneaky light mechs? Or are you looking for something unique that is low point value? Cause Firestarters are an undervalued asset that can cause fire and smoke for non-firepower related support as well as overheating other mechs that might dash in and cause a nuisance. Plus the can easily have jump jets and are fast, giving good recon.

Urbies have charm as a walking gun. They aren't the worst mech in the game (looking at you Charger) but it's not great. As rear guard it's not ideal but if you have BV to burn, try it.