Just curious, why are you not just using a simply pump jack system like I see so many of the siding guys using? I have seen new jacks which use 4X4 veticle posts available for as little as $100 each. I bet used ones could be found on e-bay.
I don't know if they are still jobsite-legal, but pump jacks were made for this kind of work. When used correctly they are relatively safe, but I'd worry about them if left in place for more than a couple weeks. There are better ones which use metal poles- those I trust well. Having a wood-pole pump jack under you is an interesting experience when they're right- they can slip when moving and they wiggle around. Having a pole break under you will get your attention- you can yell louder than you thought you could! If the top bracket is secured well it might hold you- it did me- but plywood ones will sent you down to earth which is why I won't go there anymore.
On any scaffolding you need to know what you are doing and do it properly. If you don't do that, don't get on it!
wood leg pump jacks are indeed illegal they have been for some time, atleast in canada. i still see the odd set around town but their not set up their laying in the back of guys shops buried under clutter. there are huge fines for using them. same goes for ladder jacks which are special brackets that hook up to a laddder to which a plank can sit on
OK, now I have to ask what is wrong with ladder jacks? I used them for years and never had a problem. Granted, they stress the lower rung upon which they lever, but if you are using strong aluminum or fiberglass ladders, I see no problem. Of course, you are walking on a foot wide plank without a safety railing. That may be the safety concern, rather than the jacks themselves. I have fallen off a plank a hundred times from a height of 2 feet, but never come near falling from 15 feet! Guess it is just being respectful of the potential hazard.
no saftey rail is specifically the problem as far as i know. i just know a few guys that have been forced to destroy em by the saftey cops