The standard way for doing POST is a dojo.io.bind with something like
method: "POST",
content: {
somevar: somevalue,
somevar2: somevalue2
}

But what if I wanted to create a POST request wherein I didn't yet know what all the data that was going to be passed (instead of a known "somevar" I might have multiple items which I don't know the names of at the time...)

With GET this is easy, you could append to whatever your URL is with

url+="&somevar=somevalue";