Probably not exactly what you're after but at least it won't interfere with normal operation during the day. Depending on your work environment it may be better to aggressively mine for shorter periods when the PCs are not in use at all, versus trying to throttle for the whole day.
You can run processes with lower priority in the background, so it won't interfere at all with work.
The Watts/MHash/s ratio of CPUs however is that bad, that currently you cannot even pay for the electricity used (depending on the rate of course, but well... 3 MHashes/s give ~3 Bitcents/day (if computers run during the night) currently) with the BTC generated. Did you tell this your boss too?