A lot of them are more "inductive" than "radio" and work by magnetic coupling rather than electromagnetic coupling.

Put a coil outside the metal body of the device and not too far from the skin. Then have another coil that you place on the skin, in the same region as the under-skin coil. Run a 1kHz sine wave through the external coil and you'll make SOME voltage on the internal coil. That allows you to charge.

For bonus points you can also run a communication protocol at say 100kHz (gotta get a couple of decades of frequency difference) and since you're charging the device, you can afford to "waste" a lot of power to get the signal out.

I don't do this stuff myself, but my dad did for quite a number of years.

Yep. NFC is wonderful for that sort of thing. And better than radio since it's inherently shorter range (O(r^-4) versus O(r^-2), for the same reasons as active radar). But still not perfect.

They also need to be "able to kill you" so that the patient can be defibrilated if their heart stops beating. If you do that when their heart is functioning it tends to have the opposite effect.

Right. But that functionality can be airgapped from the readout parts.

Some sandboxing could be done, but if an attacker roots a pacemaker or an insulin pump it will be extremely difficult if not impossible to prevent them from convincing the device to perform its intended function at an unintended time.

Airgapped. Not just sandboxed. As in two separate processors, with no overlapping RAM / etc.

You can't do your example, because there's no way to get at the internal clock from the radio.