How can leagues that traditionally run longer than 10 weeks be sanctioned?

Simply sanction multiple league sessions to cover the full length of your league. For example, if your league runs 18 weeks, just sanction two 9-week league sessions to cover it. The main reason for the 10-week limit is so scores get reported to the PDGA within 3 months of the time a league session starts so players can get official ratings for their league rounds without waiting half a year.