Prospect of pro-bono/voluntary work with current advisor after graduating with PHD
I think it really depends on how long you plan to work without salary. If you apply for some grants in the next months, and you'll get the answers for instance in February 2012, I guess it's ok to work without salary for a couple of months, but only because you're expecting something specific. But I wouldn't recommend being in such a situation if there are no concrete expectation, because, in addition to the fact that you might need money to live, it can also be psychologically difficult, for both your advisor and yourself (maybe it's just me, but I would feel bad asking someone to do some work without any salary, even if this person is willing to). 

You say that visa regulations make it difficult to move from one country to another, I don't know which country you come from, but having a PhD usually makes the process quite easier, especially coming from the US. You still have more than 6 months to find a postdoc, you would be better off with trying to get a job somewhere, meet new people, do something different and in the mean time, work with your current advisor on a proposal that would allow you to come back with some funding. An important aspect of a postdoc is to demonstrate your ability to cut the apron strings with your advisor, and to show that you can work with other people.