Using powdered egg shells

anything?

There's only one veg I use powdered eggs shells on, and that's my tomato/chilli's when planting them into their final stations.
I fill up my containers with the usual compost, and then mix in a good scoop of calcium powder into the hole for the plant, just gently mixing it in.
I've found that it helps prevent blossom end rot on both my toms and chilli's!

Can you add the to water for watering in situe to ward of BER or is it better to do when planting in final positions?

Good question! Honest answer is I don't know, it certainly couldn't do any harm!

On the subject of "too much lime" I will share the following.

I get decent tomatoes here in my Georgia home. However, I recall them doing much better in the garden my parents had, in Indianapolis (center of the State of Indiana, where the soil is "glacial till" (soil developed on pulverized rock, ground up by huge glaciers). Out of curiosity, I looked up my parent's soil type in a national soil database; the soil is 50 % calcium carbonate (lime)! Northern Indiana limestone rock pulverized by glaciers and pushed south.  It's no wonder my father was able to grow over 100 tomatoes on a plant one year! Compare that to the dozen (or less) tomatoes per plant on my Georgia plants grown on acid soil , but given pelleted dolomitic lime.

Apparently it is difficult to provide too much lime, more is better.

I can’t seethe benefit of adding crushed egg shells to water tbh, as if you added them, mixed it up & watered onto plants, you might as well just crush,sprinkle on the soil & then water. I’d think in the water they’d just stick to the sides of the can etc , it’s not as if you’re suggesting soaking them or something. Have you seen it advertised somewhere & with what rationale?

Crushing eggshells won't make them soluble so little point in that.  I'd just throw them on the compost heap, it's unlikely they will make any difference being applied any other way.