There is enough of this traffic to cause heartburn for the many family friendly sites that link to the expired domains.
It seems to me that sites that made it into the google directory would continue to show there for at least a little while, and you could check their pagerank there. Might need a program to crossindex expiring domains for pagerank.
However, google is showing the URLs in searches even though the URL is not active. So, that's good for purchasing new domains cheap and pointing them to your existing sites.
I suggest that you do not bother with pagerank per se. Better to look at inbound links or link popularity. With link popularity you will get pagerank when you acquire the domain name.
You will not keep your pagerank if you do not immediately publish relevant content. But you will still have traffic. And if you know what to do with traffic, then you might have a winner.
I used to have a massive spreadsheet for all owned domain names and domain names taht I was tracking. Now I have an account with namestead. That service tells me the exact day the domain name drops and then I setup use pool to catch it.
I catch about 1/2 with pool. Some of the big boys either outbid me (thousands of bukcs) or they use some of the more expensive services to just take it from everyone.
Working pretty good so far for me.