Most internal watchdog timers found on microcontrollers contain mechanisms to prevent an inadvertent feed, usually requiring the write of a specific key-value to the feed register, although some watchdog timers may require a more specific sequence (of multiple key-values written in order to the same address, or writes to multiple addresses).
However, not all applications can use an internal watchdog, for reasons that are detailed well here, and thus use an external watchdog instead. However, these external watchdog timers are fed by toggling a single pin on the IC. Simply hooking this up to a port pin is a solution I perceive as mishap-prone, where a write to other pins on the GPIO port could inadvertently feed the watchdog. While a windowed-watchdog provides some defense against premature feeding, relying on it as a sole defense against such is unwise from a defense-in-depth standpoint.
What strategies can be used to prevent inadvertent feeding of an external WDT, given that currently available external watchdog ICs use a single pin for their feed input, instead of a serial or parallel bus input? Hardware strategies are preferable, although software defenses against inadvertent feeds can be useful in a defense-in-depth strategy as well.