Well, as a fellow Heroes of the Storm player, let's see if we can find any parallels in the LOL JG camps and the Mercenaries often seen on the many myriad maps of HOTS and if such an idea is feasible.
Usually there are 2 types of mercenaries.
The Bruiser Mercs, and the Siege Mercs. Some might have Mercs that apply to that particular map's design, like the Overwatch Map Hanamura or the Volska Foundry. Others just have redesigned versions replacing the Bruiser or Siege mercs (Like the two Diablo maps Infernal Shrines and Battlefield of Eternity replacing the 4 Knight Bruisers with the Shaman and his pet Tigers/Lions and the 2 Trolls/Goldems wielding rocks, cannons or something else with the 3 Goatmen wielding spears) Then the Haunted Mines map (Can't remember the name off the top of my head) and the Towers of Doom have the Pumpkin Heads and the Headless Horseman. The Headless Horseman acts like a boss, like many of the other boss camps in HOTS, and one can assume their purpose is similar to LOL's Baron Nashor.
Most of the non-Blizzard game maps (Like the Spider Queen's tomb, Raven-Lord's Keep, Sky Temple) use the standard 2 types, along with a Boss camp on opposite ends of the map. Both are really meant for pushing, one just does it at a distance (Siege) while the other fights up-close and personal (Bruisers).
Splitpushers in League (Like
and I'm sure there are more but can't think of them off the top of my head) usually are able to push well enough on their own. Yorick has his Mist-Walkers and Maiden, for example. They kind of act as Bruiser Mercs would in HOTS. Adding in the pushing power of something like the Bruisers or the Siege Mercs might make Splitpushers nigh unstoppable. Then add in Rift Herald and
? Turrets would be falling left and right and the team facing that would be crying for mercy!
Then, let's look at how the Mercs can be stopped. Specialists, like Murky, Gazlowe, Azmodan and Sylvanas excel at stopping Mercs as well as taking them. But no champs in LOL have abilities that can match that, especially on the speed at which they can do it.
Next, I'll talk about the maps. HOTS has a MUCH larger selection of maps than LOL does. LOL has SR, TT and HA (Howling Abyss) along with the occasional Rotating Game Mode that uses a non-standard map (Like Star Guardians, Dark Star, etc). Most of the maps in HOTS are standard 2 or 3 lane maps, while LOL has only 2 mainly-dedicated maps, one for 5v5 and the other for 3v3. Howling Abyss is more akin to the Brawls in HOTS IMO.
Also, we need to keep in mind that HOTS does not have a dedicated "JG" role. Sure, some of the specialists, like Murky, maybe Abathur and a few others I can't think of have those "Bribe" skills in their talent trees that make it easier to get Mercs, but that's not the same as Jungling as it is in LOL. Ganks in HOTS are not the same as Ganks in LOL. The people who gank in HOTS are those that often have Invisibility, like Nova or Zeratul, because they are Assassins. Also, nearly every character in HOTS has at least one mobility-like ability on one of the standard abilitys of Q, W or E. Many of the champs in LOL would KILL to get the same level of mobility of someone like Genji, for example. It's much easier for characters in MOTS to escape ganks than it is for Champs in LOL.
So to sum it all up, I'd say: No. Mercenaries are NOT a feasible idea for LOL. After all, we play League of Legends because it's a different experience than HOTS, or Smite, or DOTA 2 are as a MOBA. Keep copying one another and you'll hardly be able to tell which game is which.
I can see where you're coming from this, I really can. Like you say in the post, Mercs CAN be good when used effectively. You even point out a similar idea was used in the Teemoing mode. But remember, that was a Rotating Game Mode, not a permanent playable map. Adding something like this to the permanent maps of SR and/or TT can have BIG repercussions.