Given the frequently quoted 6km/second, how many tungsten dart equivalents to a Tallboy could a Falcon Heavy or BFR put into a polar orbit?

Simple math. What is the mass of each "dart?" What is the payload capability of the launch system? Knock off some good percentage of the total payload to serve as the bus and de-orbit system, and there ya go.

There seems to be little point in going for tactical nuke yields

Little possibility of it, either. Total "yield" of the kinetic weapons is substantially less than the total yield of the chemical propellants in the booster required to orbit them, and few boosters have enough propellant to be classed in the tactical yield capability, unless you're talking about pathetically small yield devices like Davy Crockett. Starship/Super Heavy has 3.4 kilotons of propellant in the first stage, 1.2 kilotons in the second, total of 4.6 kilotons. Some sizable fraction of that is simply tossed away due to gravity and aerodynamic losses. More lost due the mass of the buss and de-orbit systems. More due to drag losses to the weapons.

Starship is supposed to have a payload of 100 tons to LEO, at *about* 7.8 km/sec for a kinetic energy of the payload being ~3E12 Joules... or about 0.727 kilotons. That's for the *entire* payload. Cut the mass in half for bus/de-orbit, and you're down to 0.36 ktons. Split it, say, ten ways, and you're down to 0.036 ktons per weapon. Shave off some drag losses, and you're perhaps 0.02-0.03 ktons per. From 4.6 kilotons of explosive power on the pad, you're down to less than one percent of that per weapon for ten weapons.

You can do better by making the Starship stage fully expendable, but you're still tinkering with tiny percentages.