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Seem like the date/time related lookups only works with dates and datetimes, not the time fields, although seems like it would be a nice feature to have as well.
Have you thought about raising a ticket maybe?
Maybe you can consider this as a replacement:
DEVICESCHEDULE.objects.extra(where=['SUBSTR(time, 1, 2)=%0.2d'], params=[nowHour])
DEVICESCHEDULE.objects.extra(where=['SUBSTR(time, 4, 2)=%0.2d'], params=[nowMinute])
Source:stackexchange.com