Liberapay is NOT only for people doing open-source projects. There is no requirement for you to release the source code of your comics. However, from their FA

Liberapay is NOT only for people doing open-source projects. There is no requirement for you to release the source code of your comics. However, from their FAQ: "Liberapay is only for donations, meaning that transactions must not be linked to a contract nor a promise of recompense." So you can't provide things specifically only to your patrons.