Condoms can be challening with karezza, because erections may come and go as the sexual connection ebbs and flows.
A woman contributed this insight:
The female condom would be perfect for karezza as it fits the woman, not the man, so no worries about erections coming and going. I'm sure the delicious energy flow could still be felt (it's very strong when your minds on really focused).
Also, I have lots and lots of lubrication since we started karezza. Most of the time we think it's a combination of me plus him (not ejaculate, but pre-cum~~which should not contain any sperm unless you have ejaculated quite recently).