Is it legal to assert a patent against an individual who has no "registered" business entity?
I'm not a lawyer, but I believe you are just as open to a infringement suit if you are an individual as a registered business. Indeed as an LLC you might be less of a target since that structure protects your family assets and makes you a less lucrative target.
If so, is there any precedent whereby an individual has been sued by a patent troll?
I'm not aware of one, but it is a well known tactic of trolls to target small business users of "infringing" technology. The idea is that small businesses using the technology are more likely to settle for small amounts than fight a suit compared to suing the large manufacturers. This page makes the following statement supporting the likelihood of suits against individuals.
Unlike operating companies that use their patents to produce and sell
their product, patent trolls often acquire patents cheaply from
bankrupt companies and do not use those patents in operations, but
rather charge licensing fees to other businesses and individuals that
appear to infringe upon a patent that they own.