What about ground covers that flower (e.g. dianthus)? I understand that a little bit of an overhead shower is needed now and then but ground covers are just too low and wide spread to be watered from the base so they'll definately end up having wet foliage and flowers with every watering session. Won't that do harm?
No, they'll be fine, mostly, assuming they're in the ground, especially if they're perennials- don't forget they get rained on quite naturally anyway.
Some plants like mediterranean herbs, some of which may be used for ground cover (Thyme, Oregano) don't really like being too wet at the root, but having wet foliage periodically is not an issue, so long as it's not every single day for months and months, and particularly if the temperature is pretty cool. Cool or cold weather and permanently wet foliage/flowers is not a happy combination and likely will give rise to fungal infections.