Bumping thread to report that I bought two one-way tickets via AMEX:
Outbound: Original coach fare of $150 was not credited; however, I rebooked for a higher, upgradeable fare applying those funds, with the $64 difference being reimbursed.
Return: I purchased $75 worth of gift certs using my 3x AS Visa, added that to my wallet, and purchased a ticket worth $172. The $97 difference was reimbursed.
AMEX says I have $20 remaining, so off to their store I go for tschochkes.
Seems like the system does, indeed, have a $100 limit regarding ticket purchases.