If I ordered a commission from someone, I'd personally think it'd be more fun if I didn't get to see it until it was completed. :> But I agree. I think they should ask/you should charge if they want to see it progress.
It might be more work than you need, considering you already have an LJ and such. Maybe use the Google list sort of thing to link to any progress shots you might have? don't know if you could do that, but maybe it'd be more convenient? xD
Honestly, creating an LJ seems a bit unnecessary. It would be very nice to have updates, though. For the LJ, it really depends on if you think you would find it too tedious to have to log in, update/post, what not, every time you wanted to post an update.
I think it may be easier to just have a copy/pastable list that you could stick in the end of each journal post (Not sure how often you update journals, though, sorry-) or even just make a periodic journal. Constant, instant updates aren't necessary, but one every few days, maybe would be nice.
It greatly has the potential to become more work than you need, and I can see a few issues. What do you do currently? E-mail? That keeps it private until the commissioner is ready to have it shown, provided they even want it public. If they're on LJ and don't want it public but would like updates there, you'd have to create a filter just for that person to view, which can quickly result in a lot of filters if you have a lot of people who like to keep private. You'd need to store the file somewhere to link it in the LJ, where in an email you can keep it on your computer and send as an attachment, unless you share your progress images as smaller than the final product and have them thumbnailed somewhere and already go with a link.
It seems like a lot of added steps, but it might provide a better way to communicate through the threaded comments, and could open it up to critique and redlining from others if you're looking for a bit of help on something, too. If someone manages to not have an e-mail in this day and age, it's a possible place for them to check to view it, but you could do the same thing with scraps here on dA, perhaps? You'd also be able to post edits or what have you by either editing the post itself and changing the image, or putting it inside of a comment reply to the original post, or starting another post all together, but that might end up as difficult to track, and result in a lot of tags on LJ too!
I dont know what your commission load is, if it's relatively light, this might be a good method. If it's pretty heavy, this might not be such a good idea. Pros/Cons!