Was SSI's isometric RPG "M" ever released?

Was it released?

No. See felipepepe post on 2015-01-21 with information from David Shelley of SSI:

Internally, we decided to wrap up the Gold Box games with Pools of Darkness (in-house), while we ramped up to do Dark Sun, and a sci-fi RPG named M. This was a turbulent time for SSI and M ended up cut.

...

It was at the cancellation of M and the revising of Dark Sun, that SSI jettisoned most of its internal development team, and positioned itself for eventual sale.

Any box art, reviews, screenshots, gameplay footage, etc?

Yes! A 2017 English Amiga Board thread 'Unreleased SSI AD&D game "M" from the early 90s' referenced several magazine previews:

• Power Play, September 1992, page 15, German text with screenshots:

  For non-German speakers: "M was not an AD&D game, and it took place in a sci-fi universe where you had to rescue some diplomats with your party of four plus one robot on the planet "Monsoon". The article mentions that some of the monster sprites were made from 3d renders which was a fairly new concept back then. Incredibly, it also says that the game was to be released at the end of the month (Sept. 1992) for MS-DOS and early next year for Amiga!"

  

• The One for Amiga Games, issue 42, August 1992, page 102:

  Already a contender for the shortest game title of the year (if you can think of a shorter one let us know), M is the latest role-playing adventure from those most prolific exponents of the art, SSI. Obviously not wanting to be left out in the cold by the new itde of more sophisticated RPGs (Eye of the Beholder II, Ishar and Ultima VI among them) SSI has cleaned up its act considerably for this newie. Gone are the bland PC-style graphics with pathetic little windows, replaced by a swish full-screen 32-color scrolling arcade-style 3D isometric display! (Phew).

  This is what RPGing in the 90s is all about - but although the technical spec may have changed, the OTT scenarios haven't. In M, the player leads a resistance group called the Midnight Raiders, who battle against the evil Terran Overlord Government on the remote planet Monsoon. Your objective - to rescue the members of the diplomatic faction kidnapped by the TOG as bargaining chips with which to achieve their foul ends.

  Most of the traditional RPG themes are still in evidence - the player controls a party of four humans and a robot, with the computer-controlled characters revealing more about themselves as the story progresses. You can chat with the various NPCs (that's non player characters to the uninitiated), included alien races, and of course do battle with a plethora of futuristic weaponry.

  But what actually is M? That's something that's only intended to be found out when you unlock the secrets of the game's convoluted story-line. M is being hailed by US Gold as "the most visually stunning, technologically advanced, intricate and compelling science function fantasy role-playing game ever!" But then they would say that, wouldn't they?

  

• Amiga Format, issue 37, August 1992, page 42:

  A quick peek at US Gold's Work In Progress reveals a number of new games shortly to come from their American colleagues SSI. First up is the snappily titled M, an isometric-perspective role-playing game with a science-fiction feel. You lead four characters (which you create yourself) and a rather useful robot through a host of dangerous locations where you meet and converse with potentially deadly aliens. And why, pray, are you subjecting yourself to such risks? Because it's your job to rescue a group of diplomats who are being held hostage by an evil government. We say, let 'em rot!

• PC Review, issue 9, July 1992, page 10

  ...Midnight Sons, uses SSI's new grapzhics[sic] based role-playing system to tell the tale of an undercover team who travel to the distant planet of Monsoon to free a group of kidnapped diplomats.

  This mission, which takes place in glorious 256-color VGA, involves the player controlling four characters and one robot in an attempt to wrest the whole planet from its captors.

  Midnight Sons will be available in July (no price has been specified as yet)...

• Zero, issue 34, August 1992, page 87:

  M: This Science Fantasy role-playing game is set on Monsoon - a mysterious planet covered by a noxious cloud. The only cities exist on plateaux that jut beyond the miasma. With the aid of a point-and-click interface and a full-screen isometric display with animated characters, the game lets you create and lead a party of four people and a robot. The plot has you rescuing kidnapped diplomats, discovering the evil plans of the bad guys, and using a lot of Glade Air Freshener.