What are the appropriate components of the ASU for wear of a SPC at his wedding (specifically, the ceremonial belt)?

There is no such thing as 'regulations for a wedding'. There are uniform regulations - period.

Chapter 13
Service/Dress Uniform-Male
13–1. Authorization for wear
The male service/dress uniform is authorized for year-round wear by all male personnel. When prescribed by CTA 50–900, the service/dress uniform is worn as an organizational item. See DA Pam 670–1 for uniforms currently classified as male service/dress uniforms and for authorized variations of the uniform.
13–2. Composition
a. The male service/dress uniform consists of: (1) Coat.
(2) Trousers.
(3) Shirt.
(4) Necktie.
(5) Undergarments.
(6) Belt.
(7) Headgear.
(8) Footwear.
(9) Socks.
(10) Undershirt.
b. The male service/dress uniform has multiple variations:
(1) Service Class A: All components are worn.
(2) Service Class B: All components are worn, except the coat; the neck tie is optional if the short-sleeve shirt is
worn.
(3) Dress: All components are worn; the bow tie is substituted for the necktie.

13–3. Occasions for wear
a. All-purpose wear. The male service/dress uniform is authorized for wear on and off duty, on and off the installation, unless restricted by the commander. The male service/dress uniform is not intended for wear as an all- purpose uniform when other uniforms are more appropriate.
b. Approved wear. The male service/dress uniform is prescribed for year-round wear for all Soldiers, unless otherwise directed by the commander. Soldiers may wear variations of the male service/dress uniform:
(1) On duty when prescribed by the local commander.
(2) At social functions of a private or official nature, either before or after retreat, and while in transit to and from such functions. The male service/dress uniform is normally considered appropriate for social or official functions off the installation, such as memorial services, funerals, weddings, inaugurals, patriotic ceremonies, and similar functions.
(3) When designated by the host of an event.
(4) On other appropriate occasions, as desired by the individual.
(5) Soldiers may wear the male service/dress uniform for commercial travel IAW paragraph 3–7b through c. c. Restrictions on wear.
(1) Personnel may not wear male service/dress uniform in off-post establishments that primarily sell alcohol. If the
off-post establishment sells alcohol and food, Soldiers may not wear the male service/dress uniform if their activities in the establishment center on the drinking of alcohol.
(2) Commanders may further restrict wear of the male service/dress uniform IAW paragraph 2–6c.







https://www.army.mil/e2/c/downloads/337951.pdf