Many years ago when I was at BskyB we had a device called a 'ladder mate', which, i must admit, was as good as anyone footing your ladder.
Although not recommended, as a demonstration of how effective the device was at footing you could free stand a 12 rung single section with a ladder mate and walk 3/4's of the way up, ladder freestanding!
Don't know what the current legistlation makes of these devices tho, and they were subject to having a level grounding. also, if they were used incorrectly you could end up with a triple section 30ft ladder freestanding from the elevation that you were looking to work from! They were good for reducing ladder bounce tho. -- maybe for the trained ladder user methinks. - you can imagine the time that the correct setting of these adds to a job as well.
A sound risk assement and method statement sounds like it will indemnify your client sufficiently for you to work at your own liability.
Good luck!
Grant.