Since no /pet hold on available due to client limitations I think it'd be better to just have it in its current state of the command always forcing on. It's extremely rare that I can see pet hold being toggled off would be more useful since the commands primary creation/use is for raiding and not having your pet go everywhere training.
Without a forced on option, things like hold not persisting through zoning, pets dying, etc will make not having a forced on option really rough on boxers. Say one of my pets dies but the other one lives and I issue my pet hold command. Now the fresh pet will go onto hold but the previously already held pet that did not die will go to non-hold. As a result instead of being able to 1 button press to put my pets safely on hold I will have to find my way to that specific window to hold it from the screen. Which causes a mild annoyance especially if boxing several pet classes.
Other than that everything looks great.