Per our PCI/Risk exposure reviewer, the ado.net appender in log4net is a risk. Essentially, if somebody can gain access to the config file, they can change the config file to run any query via an error.
Obviously, there's a bigger concern if somebody can change a config file.
The reviewer felt that with log4net being a popular tool this was a high risk cause of how easy it would be for an attacker to change it.
Other logging tools make a call to a hard-coded stored procedure to log to a database.
If the ado.net appender could be changed to call a fixed stored procedure and perhaps pass parameters with some fixed and maybe a concatenated string for a variable number of parameters, the risk would probably be removed. The SP would be responsible with working with the concatenated string. A formatter may be the way to go to make the concatenated string.