What would be the best / most stable / most cost efficient way to synchronise video(s) on other PCs to Sonar projects running off a Sonar playlist?
 
I would want videos to be running off a 2nd/3rd PC so that the main Sonar master running the playlist with backing tracks does not get more load.
 
The envisioned application would be (a) having videos synced to live performance behind the band and (b) create something like a teleprompter for vocalists ... and potentially having both (a) and (b) active at the same time.

You could try using MTC to sync up two instances of SONAR. Set one to master to generate MTC (Preferences/project/MIDI) and the other containing the video to slave to time code.
Disclaimer - I haven't tried this scenario myself but I believe the second instance should sync the video to the incoming timecode.

Thanks Noel. I'll certainly try your suggestion. Master/slave setup of 2 Sonar versions also interests me in terms of  whether it is possible to record a project with large track count into 2 PCs (e.g. in case of limited audio inputs at the audio interfaces or disk write capability bottlenecks at high sample rate projects) and then merge tracks for mixing.
 
As regards (b) of my OP (i.e. creating some sort of a teleprompter for lyrics): I found a rather elegant solution using only lyrics dumped into PowerPoint, formated quickly, then saved to PDF and now PDF files being automatically opened, stepped through and closed using only 3 MIDI messages and BOME MIDI Translator Pro 1.8 (requires purchased version).  Even works fine with playlist.  However, since I do not want to advertise for 3rd party software here, anyone interested in how I created the setup, feel free to PM me.

You can certainly use two or more PC's to record sets of tracks simultaneously. Its arguable whether you even need to sync them in that case. Most high quality audio interfaces have pretty solid clocks so its unlikely you will see much drift unless you are recording really long sessions. Once you have the recorded audio you can just import the waves into a single project, line them up and and start from there. I've had pretty good luck with even recording with multiple portable audio recorders and cameras. There is hardly any perceivable drift.
You could sync the PC's while recording but it would require more setup obviously.

Again a great suggestion, thanks.
 
For the sake of comfortable recording I would want synchronised start/stop/move between markers, but as you say, for that I don't need PCs fully synchronised. Sending both PCs the same MIDI command key bound to required Sonar functionality would do ... coming to think of it: I've used a MIDI floorboard to control Sonar from the rec room before, and if I just send those MIDI messages to either DAW, all would be set up ... great!!!