Is it permissible to delegate someone to pay charity on my behalf in my home country, then reimburse them the full amount at once?

There is no impediment to appointing someone else to give out charity on your behalf, whether they pay it from their own money to be reimbursed later, or they pay it from money of yours that they possess, and whether the charity is obligatory or non-obligatory. The debt must be repaid promptly if they paid it from their own money.