[JetMetFan]

NCAAM Rule 1-23-1

An undershirt is considered to be part of the game jersey and must be a color similar to that of the game jersey as defined by the neutral zone. In addition, the sleeves and neckline of undershirts shall be unaltered (e.g., no cutoff sleeves or cut necklines). Both sleeves shall be of the same length and not extend beyond the elbows. No logos, decorations, trim, commemorative patches, lettering or numbering may be used on an undershirt. The same color must be worn by teammates

There is also this in Appendix VIII (Accommodations for Student-Athletes with Disabilities)

An NCAA member institution will have the right to seek, on behalf of any student-athlete with a disability participating on the member’s team, a reasonable modification or accommodation of a playing rule, provided that the modification or accommodation would not:
1. Compromise the safety of, or increase the risk of injury to, any other student-athlete;
2. Change an essential element that would fundamentally alter the nature of the game; or
3. Provide the student-athlete an unfair advantage over the other competitors.
To request any such modification or accommodation, the member’s director of athletics, or his/her designee, must submit a rule waiver request, in writing, to the secretary-rules editor. Such written request should describe:
a. The playing rule from which relief is sought;
b. The nature of the proposed modification or accommodation;
c. The nature of the student-athlete’s disability and basis for modification or accommodation; and
d. The proposed duration of the requested modification or accommodation.

Long story short, if BYU asked for a waiver and received one it would be allowed. If it's granted, BYU has to have the written ruling on hand for every game and show it to the officials and the opposing team.