Yeah, something along those lines certainly could work for man-orcs. It I were feeling really ambitious, head swaps or sculpting on very basic orc faces would probably be be feasible. It has been a while since I read the novels, how large a part of the force would the man-orcs be?
It's hard to say. But they seemed to have been significant. They are, however, introduced somewhat obliquely.
At Helm's Deep, Gamling says, "But these creatures of Isengard, these half-orcs and goblin-men that the foul craft of Saruman has bred, they will not quail at the sun".
Now, to the reader already familiar with the Uruk-hai of Isengard, the reference appears to be to Saruman's Uruks with their Mannish gear and disdain for sunlight. And, actually, it might be: Gamling isn't necessarily using the terms taxonomically, and Tolkien
loves doubling up descriptions of the same thing, so he (through Gamling) might not be even distinguishing two sorts of creature. It could be a "brigands and bandits" sort of trope.
But then, when Aragorn and co. meet up with Merry and Pippin at Isengard, we get this description from Merry:
"And there were battalions of Men, too. Many of them carried torches, and... I could see their faces. Most of them were ordinary men, rather tall and dark-haired, and grim but not particularly evil-looking. But there were some others that were horrible: man-high, but with goblin-faces, sallow, leering, squint-eyed.... they reminded me at once of that Southerner at Bree: only he was not so obviously orc-like as most of these were...."
So they were a fairly small minority of the 'Men' marching out of Isengard. But there were still lots of them:
"'I thought of him too,' said Aragorn. 'We had
many of these half-orcs to deal with at Helm's Deep."
And back in the Shire, Merry says, "Like
many I saw at Isengard" when he sees the Orcish-featured ruffians.
The best description of them comes in 'The Battle of the Fords of the Isen', where they're described as a company and are employed to break the Rohirric shield-wall, the Isengarder Orcs being ill suited to the task because of their short stature:
"As soon as the enemy had gained possession of the eastern end of the Fords there appeared a company of men or Orc-men (evidently dispatched for the purpose), ferocious, mail-clad, and armed with axes. They hastened to the eyot and assailed it from both sides."
The individuals that are seen in 'close up' are described as 'great':
"As he came to his side Théodred fell, hewn down by a great Orc-man."
"Elfhelm himself, however, sprang up towards the knoll; and there he found Grimbold fighting two great axemen for possession of Théodred's body."
So they seem to be the elite of the army: armoured like the Orcs (and unlike the Dunlendings) but much bigger than them (and Saruman's Orcs were big for Orcs). And, therefore, presumably quite a small proportion. Maybe a couple of bases' worth given your fairly large barbarian and Orcish forces?
Converting them would be fantastic - though fiddly! One other option
might be to use the
Battle Valor orcs, which are huge for 15mm (though
perfect for Isengard Uruks in 1/72). But they're probably a bit too Orcish in build: "squat and broad" rather than "tall and grim" - the half-orcs definitely seem to have been in the latter camp.