お、行けるじゃん。

と、思ってみていたら、Dev GuideのFramework TopicsにあるApp Widget項目の中に下記記述が。

The actual update is not guaranteed to occur exactly on time with this value and
we suggest updating as infrequently as possible—perhaps no more than once an hour
to conserve the battery.

Note:
If the device is asleep when it is time for an update (as defined by updatePeriodMillis), then the device will wake up in order to perform the update.
If you don't update more than once per hour, this probably won't cause significant problems for the battery life.

If, however, you need to update more frequently and/or you do not need to update while the device is asleep,
then you can instead perform updates based on an alarm that will not wake the device.
To do so, set an alarm with an Intent that your AppWidgetProvider receives, using the AlarmManager.
Set the alarm type to either ELAPSED_REALTIME or RTC, which will only deliver the alarm when the device is awake.
Then set updatePeriodMillis to zero ("0").