It used to be possible that you could set up HomeKit without iCloud syncing, but only if your iOS device did not partake in any iCloud synced services whatsoever (no Photo sharing, no contact/calendar/reminders/notes etc. syncing enabled.) Remote access also didn't work in this scenario, it would only be your iOS device talking directly to a HomeKit device while connected locally in the LAN or directly using Bluetooth.
But the last time I tried this, iOS 8 was the currently shipping iOS version. May I ask why you're interested in disabling the iCloud keychain?