Purely anecdotal evidence, but I'm a native English speaker, well educated, and pretty literate, and "omnibus" meaning "multiple novels in a single book" is one of those definitions that I had to think about a little while before I said to myself, "Oh yeah, I think I've seen it used to mean that once or twice."
I don't think it's widely-used enough that you could expect a general reader, even a well-educated, literate reader, to be familiar with that definition.
Like any word that might be unfamiliar to the general reader, you can use it if you briefly define it on first mention. Like, "An omnibus -- a collection of novels included in a single volume -- is commonly published when ..." or whatever. In this case the definition needn't be long. It's not like you're trying to refer to some very complicated topic that would require a long explanation.
But I wouldn't expect a reader to just know what it means.