Not long ago I had a potential customer ask about a cue extension. I got a very good suggestion from Chris Hightower to not add the extension to the end of the butt, however, to make it so that it fit in between the butt and the shaft. The potential customer didn't like that idea, so I'm pretty sure he went to Chris Byrne to build him a brand new 62" cue. I'm sure it turned out (or will turn) out to be a great playing cue and gorgeous to look at as well. One extra expense for such a long cue that some don't think of is the extra long custom case that will need to be purchased to carry it in. But, if in the end, customer gets what he wants, then it is all good.
I have seen other players use this conical sleeve thing that they slide down the whole cue from the tip end until it snugs down on the butt. It's made of lightweight plastic with a soft inside liner of some sort, as near as I can tell. It doesn't sound like that what your customer wants, but I thought I'd throw it out there.
Good Luck!
Gary