That rule is long since not a proposed rule and is in the FEI rulebooks. Blood is VERY specifically mentioned and it was used to eliminate a competitor this week at the Olympics. Horses are vetted before and after tests, and the ground crew can now contact the judges mid-test if necessary to stop any suspected abuse.
7.6 Bleeding:
7.6.1 If the Judge at C suspects fresh blood anywhere on the Horse during the test, he will stop the Horse to check for blood. If the Horse shows fresh blood, it will be eliminated. The elimination is final. If the Judge through examination clarifies that the Horse has no fresh blood, the Horse may resume and finish its test.
7.6.2.1 If the FEI Steward discovers fresh blood in the Horse’s mouth or in the area of the spurs during the check at the end of the test (Article 430.9), he informs the Judge at C, who will eliminate the Horse and the Athlete.
7.6.2.2 If the FEI Steward discovers fresh blood anywhere else on the Horse’s body (i.e. other than the Horse’s mouth or in the area of the spurs) during the check at the end of the test, the Athlete/Horse combination is not automatically eliminated. The FEI Chief Steward shall inform the Judge at C whether the Horse is fit to continue in following competition(s) in the Event. The Judge at C will take a decision on the Horse’s fitness to continue, based on the FEI Veterinarian’s advice. If the Judge at C decides the Horse is not fit to continue, the Horse will not be permitted to take part in any further Competition(s) or tests at the Event, but the result(s) obtained by
the Athlete/Horse combination in any Competition(s) or tests already completed shall stand.
7.6.3 If the Horse is eliminated pursuant to the above, or if the Horse is injured during the test and starts bleeding after finishing the test, it should be examined by an FEI Veterinarian prior to the next Competition to determine if it is fit to continue in the Event the following day(s). The
decision of the FEI Veterinarian is not subject to appeal.
So yes -- you could in theory abuse your horse in the name of dressage. You'd be caught out pretty quickly; if historical, in the pre-vetting, and if current, during or after the test.