Well, IMO welding it back in would make it look dodgier than just leaving it out but keeping hold of it but not having it attatched!

i'd make a distinctive mark around the number while its still on the car, take pictures of it still on the car and rust spots to show why the pans being replaced, prefferably the marking and rust in the same pic but with the number visible too if possible and take photo's of the work till it's finnished, and a photo of the vin number cut off the old pan, or even keep the whole old pan intact if you can? that way when it's being sold, the new owner will know the score instead of being dubeous as to why it's not their!

Even a digital video of it being cut off etc would be good!