Left hand lock requires wood routed out of the wing, two posts installed, a tab soldered to the whisper key. It's not simple work really, although plenty of techs can do it.
My general opinion is to forget adding a left hand lock, as what a tech will have to charge to install it isn't probably worth it.
I've had extensive time on bassoons with either a left or a right hand lock (working at any given time) and currently have both functional on my bassoon. While it's common for German setups to have the left hand lock and I usually agree with German keywork choices (like alt low C for instance) I think Americans get this one right with the more common use of the right hand lock. I can say that I use my right hand lock about 99% of the time I use a lock.
A better choice for bassoon24 is to upgrade the right hand lock from the silly plunger style lock that is the default option on the 220 to the Fox rotary style lock. It's easier to engage, disengage, and get to in a hurry compared to the plunger style one. It also doesn't require likely any additional modification to her bassoon - just swap out one lock for the other - the screw holes are even the same I think.
I hope I'm not overly dominating this conversation. I just think about these things a lot (like it's my job or something) and have come to a lot of hopefully well rationalized opinions on these matters.
M.M.A., D.M.A. University of Illinois at Urbana/Champaign: B.Mus. Lawrence University
Bassoon professor at University of Wisconsin Eau-Claire
Maker of the Little-Jake electric bassoon pickup and Weasel bassoon reeds