While nothing is stopping you from putting a SAG Mek with a unit of Big Gunz (or really any unit as he is an IC), there are two main things to consider; Effectiveness and how high of a threat will it be interpreted as.
So lets take a look at Kannons, the basic Big Gunz artillery unit. At first glance this looks to be a good choice as Kannons are going to usually be aiming at Vehicles or blobs of Infantry should they be a juicer target. Sure the statistical average of a SAG is Str7, which isn't bad at AT if you're aiming at anything less than a Land Raider or Monolith, but it's random and an Ammo Grot doesn't let you reroll Str, only the pieplate placement. Should the Kannons be aiming at AV 13 and you roll under the statistical average, you've pretty much wasted the shot, unless of course you get a nice Scatter and hit something nearby. But then you also have to consider the chances of rolling doubles, such as a Zoink which would teleport the Mek into Base Contact with the Tank in question. Or the worst case scenario of Boom which would remove your entire Artillery unit, a loss of roughly around 150+ points due to a bad roll (of course options alter the points, which I'm approximating anyway). With the Mek attached it also becomes a much juicer target, especially if your playing the mission where Heavy choices are Scoring Units.
And yes, there are two other two options of Ork Artilery, but I don't think you'd want a SAG with Lobbas since the point of Lobbas is indirect fire, with which a SAG couldn't contribute, and Zzaps are already notoriously unreliable that adding another variable Str weapon to the unit seems insane.
I'm not personally a fan of adding a SAG to Lootas for many of the same reasons, it just becomes too tempting of a target for the enemy, the SAG can self destruct the entire unit any time it fires, you usually want to aim the Lootas and SAG at different targets, and unless you have a large central ruin in your Deployment Zone that can see the entire battlefield, you're better off with placing them in different areas to have a better field of vision for your Long Range Dakka.
Now I'm just giving my personal perspective, and I'd like everyone to know that they are perfectly free to debate against my personal standpoint, but I honestly feel the best choice unit for a SAG to run with is Grots. I've already said why, but to reiterate in a simple list format...
- Grots are by far the lowest priority unit in the eyes of both Ork and non-Ork players, due to their all around low skills and lack of an armor save. A SAG attached to them, does raise their profile, but usually isn't considered as high of a threat as a couple full strength Boyz units running at the opponent, or Battlewagons zooming at the opponent, or even a unit of Kans moving up the field.
- Grots are able to hold Objectives in any game, and should you have one that you place on your deployment zone nothing says you can't place it close or even on a terrain piece such as a Ruin that would give the SAG a good view of the field to fire his Long Range weapon.
- In the event that the SAG does self destruct (even on the first turn), all you lost was Grots, not an incredibly huge loss in the end, whereas any other unit would be a much larger loss and damage your overall effectiveness
- Grots are cheapest point cost ablative wounds for a SAG
- It's kinda fluffy to have a Runtherda with invisible Snotlings nearby a SAG, I mean it does fire live ammunition supplied by a Herda.
The last thing I have on the SAG at the time being is that I wouldn't put him around or in Fortifications. This is due to the mild ambiguity of the Boom result, and people arguing that a Bastian would or would not be a possible "model" to be removed from play. Personally I think the RAW (read as written) is quite clear and would include a Bastion a SAG is hiding in/around would be removed from play, but with the lack of a FAQ on it, I can see players arguing over this one long into the night.