Why not have them hold 10 isometric contractions of 6 seconds every hour during the work week. Over the course of the week it will add up to an extra forty minutes of work. Here is some supporting research.
Hope it helps.
“In the present investigation, it is not surprising that with isometric training, there was a marked increase in muscle strength. Thus, the isometric exercise program worked well in terms of increasing muscle strength. It is significant that there was a 20% increase in muscle strength with only 7 minutes of (isometric) work each day.
But the program, while increasing muscle strength was equally matched by the benefits of weight and girth loss. The loss in girth after 2 weeks, for the average person at the waist was 3.0 cm which was equivalent to one pant or dress size. After 4 weeks the loss increased to 3.3 cm. The weight loss for some subjects in the first 2 weeks was as high as 8.4 kg while after 4 weeks some subjects lost as much as 10.1 kg in body weight.
Almost 25% of the subjects lost more than 4 kg in the first 2 weeks. More than 10% of the subjects lost 5 cm or more at the waist. The circumference loss of several of the participants was 5 cm or more in the first 2 weeks.”
The Journal of Applied Research (2007)