I started to reply to this before dinner but didn't get to finish in time for first response. However Simon has said pretty much what I was thinking.

Your customer must not be worried too about money. IMO the mod will cost as much or more as the guitar. I'm not casting aspersions though, as I have way over-capitalised on numerous guitars! The one difference there is I can do all my own work.

I also do set-ups, repairs and mods for paying customers, and I have actually talked people out of doing things they thought they wanted just because it was economically ridiculous. Sure, I could take their money, but I guess I've never been a great businessman in that sense .

As for the blocking the vibrato route, I built a hardtail strat out of a body routed for a vibrato 2 years ago. I did as you & Simon have talked about except my blocking was an "L" shape as I wanted the entire back cavity filled and finished over.
That said, I used hardwood blocks and epoxy and I wouldn't think twice about hitting it with a router or worried about strength. It is solid as a rock.