SaltyDog

Here is the documented contract/UPS policy (Flight operations manual)/FAR for UPS Off Duty Flying

Contract:
It is the crewmember's responsibility to assure he is available and within FAA guidelines when reporting for work regardless of any flying activity outside of Company business. However, no crewmember shall perform flying, which would count toward his maximum flight/duty limitation under the FARs, other than that performed for the Company, except as provided in Article 10.V. This does not restrict the flying of military aircraft by a crewmember affiliated with a military guard or reserve organization. (10.V is only for Flight Engineers working towards pilot flying hours,none of these positions exists any longer)

UPS Policy (Flight Operations manual) NON-UPS BUSINESS OR ACTIVITY
Do not allow involvement in an outside business or activity (non flying) to interfere with the availability to perform duties, qualification currency or job competence. Do not perform flying that counts toward maximum 14 CFR flight/duty limitations other than flying performed for the Company. This restriction does not affect the ability to fly for a military guard or reserve unit.



§ 121.471 Flight time limitations and rest requirements: All flight crewmembers.

(a) No certificate holder conducting domestic operations may schedule any flight crewmember and no flight crewmember may accept an assignment for flight time in scheduled air transportation or in other commercial flying if that crewmember's total flight time in all commercial flying will exceed -

(1) 1,000 hours in any calendar year;

(2) 100 hours in any calendar month;

(3) 30 hours in any 7 consecutive days;

(4) 8 hours between required rest periods.
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commercial flying includes all the other numbers: Anything one would do to get compensated in 91, 135, etc

Do UPS pilots exercise their certificate for commercial flying under 14 CFR outside of UPS commercial flying?
ANS: Technically prohibited and does not even state anywhere in policy or contract that can obtain or need Chief Pilot permission as prohibited.
My opinion: In a company discipline manner, Higher management would state that a a CP does not have the authority to waive the contract or UPS policy unilaterally. If wanted to fly the G550 on the side, and wanted to stay in the contract and UPS policy, would go to the Director of Operations for permission. Surmise that individual written permission would be considered appropriate level of management to provide permission to fly other commercial operation flying.

Many commercial airlines prohibit outside commercial flying in general. Essentially, the employer is purchasing all commercial flying hours available to an employee (exclusive use). Waivers? Up to each employer.