Oops, I’m an idiot. It’s very simple.
The first thing I had tried had a typo, hence the confusion, but it was actually the right thing to do. If anyone cares, you do the same as what is said in the link in my previous post, but for each item in the array. Like so:
{
"set": {
"uid": "0x4eae",
"items": [
{
"uid": "0x29",
"items|count": 2
},
{
"uid": "0x44",
"items|count": 1
}
]
}
}