Anyone know if ther is such a stipulation at the Dept. of Labour ?

No, but I've never come across an employer willing to go to the trouble for anyone working less than full-time.

The restrictions for non-teaching jobs is monthly taxable income based on nationality, with Yanks required to earn the most, other farang countries sliding down, roughly 55K to 35K if I recall correctly.

I would think as long as that requirement is met, there's no reason for the number of hours being worked to earn that income to even be mentioned. . .