Restrict system.log logging to particular error level?

I think you'd like to set the level to Zend_Log::WARN, according to Zend_Log:

const EMERG   = 0;  // Emergency: system is unusable
const ALERT   = 1;  // Alert: action must be taken immediately
const CRIT    = 2;  // Critical: critical conditions
const ERR     = 3;  // Error: error conditions
const WARN    = 4;  // Warning: warning conditions
const NOTICE  = 5;  // Notice: normal but significant condition
const INFO    = 6;  // Informational: informational messages
const DEBUG   = 7;  // Debug: debug messages

As I see you cannot and won't go through the 3rd party code and set this globally, you could try to adjust the PHP error reporting this way:

error_reporting(E_ALL ^ E_NOTICE);

I didn't test it but maybe it helps to get rid of the entries in the log.

For advanced logging capabilities, take a look at https://github.com/magento-hackathon/Logger