Regarding the SoftClaws you're right - I'd never get the bloody things on him!!!
It's great that Jethro has you! Carpet clawing is a very common problem though, but I guess it's not so bad if your in a home you own. I'm slightly panicking now because we are actually moving very soon, and he's managed to claw up the carpet (like Jethro) corners pretty bad in two rooms...... the deposit return is looking very unlikely now...!!
I have put some old rugs at the spots, the only problem is, he does it from each side of the door, so I'm going to have to get MORE rugs! (Or like a previous post, just go to a carpet company and see if they have any scraps.) D'oh. Seems to be working though, as I can sleep easy if he's just ruining that one - it may be encouraging, but also like you, I can't catch him in the act! If I ever get up to stop him, he does, then I sit back down, and I hear the dreaded sound again. Only upstairs is carpeted, but it's where he is most happy, and when we leave the house for the day he manages to open the door from the dining room and sneak upstairs. I normally shut all the doors upstairs, but when I did this, he clawed, so now I just keep all the doors open. He's very good anyway, I thought he might mark his territory in the bedrooms, but luckily he just goes outside when we're back home.
I'm still uncertain to try spraying him, as we haven't had him long.
Some good suggestions you have, so I wish you the best of luck! I'm sure he'll gain confidence soon, it's great he has someone kind looking after him! Bloody cats, why do we love 'em??....