It wouldn't be an electromagnet. It would be some strong permanent magnets with an attachment/detachment mechanism, like an inflatable interstitial bladder or s

It wouldn't be an electromagnet. It would be some strong permanent magnets with an attachment/detachment mechanism, like an inflatable interstitial bladder or spacer jacks. The key would be to let the magnet connect and disconnect slowly, controllably and quietly. It's quite doable, AFAIK (not a magnetic limpet engineer!). As for power, why not a turbine powered generator? The host ship won't miss the few tens of watts extra drag.