Yup, you can still do something like that after GV "stops working" with products like the Obi, but it won't be free.
What you'd do is get set up with someone like CallCentric, Obivoice, or some other VOIP provider that can get you a DID (a phone number for all intents and purposes) and a calling plan (on a per minute, per month, or per year rate, etc). You then have GV forward calls to that DID and have the Obi registered with that provider.
I currently have Obivoice and they allow you to spoof outgoing Caller ID as long as you own the number (basically, they call it and have you enter a code).
Forum gurus, correct me if I'm wrong, but I think most VOIP places will charge incoming minutes for calls forwarded from GV, right?
Anyway, it's not drop-in, but close. You'll need to set up her Obi, and manage payment for the account you buy the DID from, but other than that, her number will be the same.
Cons to this approach that I encountered:
1) I had a pretty noticeable lag when talking to others. I'm not sure why, and others have reported no problems. I had (OutsideWorld -> GV -> Callcentric -> my Obi) for incoming and (My Obi -> GV -> outside world) for outgoing. I was always talking over people who called me and whom I called. It got bad enough that the WAF (Wife Acceptance Factor) went negative rather quickly.
We switched to Obivoice for everything and now it's great. Services like CallCentric and others offer different options based on your need, but they should be fine as well.
2) The "timeout" if you will that determines when GV picks up for voicemail is not very configurable. I had to set my physical answering machine to pick up after two rings to make GV not pick up (sometimes it would pick up after 3 rings, sometimes 2, sometimes 4, it was too inconsistent). If you only want GV (and not a local answering machine as well) to pick up, this won't be an issue for you, and you can just disable the local answering machine.
Alternatively, you can port your GV number to a VOIP service that offers voicemail similar to GV, but if your grandmother is used to GV, that might be a non-starter.
Hope that helps. If and when you decide to take the plunge, there are many people here who can help you out.
Later!
Jeff.