Well, I'm on my way to getting around that. Here's what I did so far:
1. I used Tasker and two plugins: AutoNotification and AutoWear.
2. With AutoNotification (using 'logs') I intercepted a notification from Zoiper (replaced the real number with many 8's):
Notification Intercepts - 2015-10-11 16:18:06.185 - New Intercepted Notification:
01. App name: Zoiper
02. Title: Incoming call
02. Title When Expaded: Incoming call
03. Text: 8888888888888888888888
04. Icon: file:///storage/emulated/0/Android/data/com.joaomgcd.autonotification/files/com.zoiper.android.app_-1037243240
07. Status Bar Icon: file:///storage/emulated/0/Android/data/com.joaomgcd.autonotification/files/com.zoiper.android.app_-1037243240
22. Action On Touch Id (Use with AutoNotification Actions): 9e5204cc-5479-4474-abe7-cf20df8cc5ba
Cancelled Or Created: Created
Id: 2
Package Name: com.zoiper.android.app
Progress: 0
Progress Indeterminate: false
Progress Max: 0
Show Chronometer: false
Show When: true
Status Bar Icon: file:///storage/emulated/0/Android/data/com.joaomgcd.autonotification/files/com.zoiper.android.app_3226745;
Licensing - 2015-10-11 16:18:06.016 - Lite from cache: false
3. So I created a 'profile' that intercepts a notification from package 'com.zoiper.android.app', with id: '2', with title 'Incoming call'.
4. When the profile (3) matches I send a notification to my watch using AutoWear, using 'priority: 2' and some vibration pattern (in order to wake up the watch).
That's all I did so far, just a proof of concept, but it works (albeit I haven't even set up passing of incoming number/name yet).
After I do that, the idea is to maybe even pick up the phone using a swipe on the watch (like the regular phone calls).
Hope this helps. Somehow I don't think the Zoiper team has any interest in directly supporting Android Wear :(