This used to be problematic, but was addressed with a change the last time the rules were updated. The definition of mark says "...an object attached accidentally to a mark is not part of it", meaning that a chase boat intentionally tied up to the signal boat is part of the mark. The definition used to say "...attached temporarily or accidentally...", which led to the problem you're concerned about.

Note that this wording of the definition allows race committee to use a "barging buoy" without worrying about how to write the sailing instruction. A "barging buoy" is an object that RC attaches to the stern of the committee boat to keep boats a little farther away. This reduces the chance of collision with (and damage to) the committee boat -- which helps reassure signal boat owners.

I hope that helps,
Eric