Reiterating:
IM Rule 5.01 [BIKE CONDUCT] (m) Except as set forth below in Section 5.02(b)*, additional equipment that has the effect of reducing wind resistance is prohibited during the bike segment of the Race (whether such equipment is worn under the athlete’s clothing, over the athlete’s clothing, or is otherwise attached to the athlete's body or the athlete’s bike); (DSQ)
* 5.02(b) covers helmets
The way ahead here is deterrence, which will minimise the need for enforcement.
A clear message from race organisers to athletes describing how this rule will be policed/enforced would help, and then informed shouty referees on the route from bike racks to mount line delivering that enforcement (DSQ if not remediated immediately). The "bike segment of the Race" doesn't start till the mount line: cross that with stuff down your front and it's a DSQ.
If a referee can give Sharpe a penalty (Boulder, cost him $$$) when a bottle falls out of his behind saddle bottle cage, then enforcing this (prohibiting equipment under an athlete's clothing which has aero benefit) should be child's play.
Challenge Roth (presumably under DTU Rules) can show the lead here.
Off topic thermo-regulation link shared by @bja : (7 minutes) and (6 minutes) and detail in his interview with Prof Heller (coolmitt.com)