Is the questioner obligated to pay, given that he issued bank checks in his name to a friend who borrowed with usury and fled, with the lender's prior knowledge of his lack of guarantee or ability to pay?

Assisting in usurious borrowing, by writing checks or otherwise, is Islamically forbidden, based on Allah's saying: "And do not cooperate in sin and aggression." Writing checks serves to document the debt. If you explicitly tell the lender that you are not a guarantor and he accepts, then you are not obligated to pay the debt. The borrower is obligated to repay the principal amount of the debt, not the forbidden usurious increase.