Under slab pin hole leak in copper heating pipe

Ive had some success with this Fernox product. It depends on the size of the hole.

The aerosol can makes it easy to push into the system

How old is the system? Once copper starts to pin hole there tends to be more coming😳 if it is a 50 year old system, start thinking of a plan B retrofit

There are products that claim "Boiler Seal" but it could do more harm than good. Especially if the boiler has a tankless coil. The sealer will adhere to the coil, drastically reducing heat transfer. Forget it altogether if it's a mod con. It will also adhere to the interior walls of everything, not just attack the leak.

If the boiler gets replaced, all pipes will need to be flushed well to get rid of the sealer.

Have the leak repaired, and try to devise a way to get the pipes out of the slab if possible. Do you have central air conditioning? You could eliminate the baseboard and add a hydro coil to the air handler. Its not something I normally recommend, but I don't think you've seen your last slab leak. If the air handler is in the attic, it might need a glycol mix.

is the heat in floor or baseboard?

Copper? Under the slab? Leak? You're going to have more leaks. Sorry about that — so it is time to figure out how to repipe the thing with a minimum of mess.

Without seeing how the space is finished off, I really couldn't suggest options — or rather, I'd couldn't make a suggestion which might work (I can think of several options…)

Are the baseboards cast iron or the slant-fin type? The slant-fins allow you to run a return pipe inside the enclosure, so you can run the supply and the return in the same chase.

You're going to need to replace the pipe in the slab. The easiest way is probably going to be to run it in the walls and ceiling. If you switch to home run piping you can probably use 1/2" pex throughout, which fits in a lot of tight spots. If the walls are block with a 1x furring strip holding the drywall, you can run 1/2" PEX in the space of the 1x furring strip. Going perpendicular to joists isn't a problem, although you do need access. If you have recessed lights often you can pop them out to get a look into the ceiling.

If doing a full repipe is too much, and you know where the leak is, you can bypass just that section. But the other pipes are going to fail in time and ultimately it's less work to do it all at once.