Is there a regulation that covers what the MP wears on his/her duty belt?

A good SSG would provide the referenced regulation. An excellent SSG would provide paragraph and line number with the referenced regulation. NOTE: Any rank can be substituted for SSG, as used here.

If the SSG is saying there is a regulation that covers what goes on the duty belt, have him or her provide it. They obviously know it or they would not mention it. It's a way to make the person put up or shut up.

Closest I could find was DA Pam 670-1 18-17 Military Police Accessories.

18–17. Military police accessories
a. Type. The MP accessories are organizational issue items.
b. Description.
(1) Badge, MP.
(2) Belt, black, load bearing, approximately 2 to 3 inches wide, with buckle.
(3) Brassard, MP, or Criminal Investigation Division (CID), subdued, foliage green with black letters.
(4) Carrier, club, or baton.
(5) Case, ammunition magazine, black.
(6) Case, first aid, black.
(7) Case, handcuffs, black.
(8) Case, oleoresin capsicum (OC), and/or pepper spray, black.
(9) Club, policeman’s with leather thong.
(10) Baton, metal, collapsible, black (or other locally prescribed baton tool).
(11) Gloves, white cotton.
(12) Flashlight.
(13) Carrier, ring, flashlight.
(14) Handcuffs, ratchet type, double lock (two-link maximum).
(15) Helmet liner, MP (ceremonial only).
(16) Holster, pistol, black.
(17) Holster, Taser/stun gun, black.
(18) Lanyard, olive-drab, black or white.
(19) Whistle, patrolman, brass, black or olive-drab.
(20) Equipment, safety, as locally prescribed (for example, reflectorized vest or inclement weather).
(21) ACU patterned equipment is authorized for wear inside an Army corrections facility.
c. How worn. The articles listed in paragraph b, above, are authorized, but not mandated for wear with the Class A,
Class B, and utility uniforms by MP personnel while performing official MP law enforcement or corrections duties. The articles may also be worn for ceremonial purposes as authorized by the provost marshal or commander. See figures 18–10 and 18–11 for examples.

You may want to check the policy letters your organization has on the wear of your uniform as well as ask the NCO who keeps referencing the regulation.