No, Bruce Heard did not get involved with The Known World/Mystara towards the end of its run.

Replica of a map appearing in Dragon #155. Replica by Thorfinn Tait.

Mystara first appeared in published form in 1981 and even existed before that as the home campaign of Tom Moldvay and Lawrence Schick in Ohio. Of course, it would take years before it got the name Mystara. While most fantasy worlds evolve over time, this is especially true for Mystara which saw significant changes over the decades when it was published by TSR. The incorporation of Blackmoor into Mystara's past is one such major evoltionary step of the setting.

There seems to be a common misconception that you can divide the evolution of Mystara into two stages:
1) The Known World
2) Mystara under Bruce Heard.

While Bruce Heard is probaly the single person who had the most influence over the development of Mystara (no disrespect to Aaron Allston, Frank Mentzer, Tom Moldvay, David Zeb Cook and others), it should be noted that Bruce did not join the development of Mystara late in the process. Bruce Heard became Product Manager of the Classic D&D line (and by extension the Classic D&D Setting, later known as Mystara) no later than 1987 with the release of Gaz 1 The Grand Duchy of Karameikos. By that time he had already published several adventures for the setting, including M1 Into the Maelstrom (1985) and CM7 Quest for the Tree of Life (1986). He also wrote Gaz3 the Principalities of Glantri that was published in 1987.

Sadly, Bruce Heard was not able to work with Mystara anymore when TSR decided to dump Classic D&D and turn Mystara into an AD&D setting in a process which lead to Jeff Grubb leaving TSR.
Bruce did continue supporting Mystara in the ways he could and did return with articles in Dragon even after the setting was effectively discontinued in 1995  with articles such as the Rakasta article in Dragon Magazine 247 in 1998. He also returned with a final installment of the Voyage of the Princess Ark in Dragon #344 in 2006.

So while an important framework for the setting was laid down with the description in X1 the Isle of Dread (1981) and with the B/X and BECMI D&D rulesets between 1981-1986, it is difficult to divide the history of Mystara as before and after Bruce Heard. Bruce Heard's influence of the setting increased gradually, just as the development of the setting itself happened as a process of gradual evolution rather than as stages of revisions under different management.

-Havard