I have access to a high powered Karacher diesel steam cleaner,

is it a good idea to clean the calipers this way, I would imagine it would remove any gunk, but could it damage anything?

No you should be fine.

On all pressure sprayers you can adjust the pressure at the wand if your worried anyway.

Get stuck into it.

I can't see it will remove corroded aluminium? But by the same token I don't see you having a problem. Well, not unless you send it flying across the workshop.