Follow up: I ended up calling and pre-paying or a temporary limit increase were not options, as others predicted. The agent did suggest that making a payment, and immediately paying it off, and repeating, could work, but that seems rather cumbersome and I wanted to get this done before Jan 15.

I submitted for an increase on my limit to $40K, but that was going to take several days to process. Then it dawned on me that a credit transfer from my other Amex might be possible. In fact, it is easily done online. I have over a $30K limit on my EDP card, but it wouldn't let me transfer any credit because I had a limit increase request in process. Another call to CS and I got the increase request canceled, and was able to transfer credit. I just made a $30K tax payment using ACI at 1.85% fee, and will pay that off as soon as it posts and make another $10K payment.