I'm pretty sure you can, I think I've read other posts of people doing this, sending payments as they can afford it. I'm not sure how you'd make sure the payment gets applied to your account, maybe make a copy of any confirmation or any paperwork you have and send it with the check, asking them to apply the payment to your reservation? Or, if you have your ressie number, just write the # on the check and enclose a note asking them to apply the payment?
Good luck, Steph