Question: I have a system which has the following structure:

Inbound > Open > Pending > Awaiting reply > Ready > Closed

I'd like to use single-words, so the one that sticks out like a sore thumb is "Awaiting reply".

I obviously can't use "Pending" because that's already in use. And "Awaiting" is too similar to "Pending".

Is there a word for "waiting for a reply"?

Answer: 
I think you may have to live with "awaiting reply." Even if you do find a single word that comes close to the same meaning, it is likely to be obscure or not appropriate for the context.

In technical writing, which is how I would classify documenting a process, it is sometimes necessary to sacrifice elegance in favor of making your writing clear and easy to understand quickly with a minimum of mental friction. In this case, your goal is to make your writing "invisible": if the reader "notices" what you've written, that means you've failed. When the reader encounters the labels you've given the process steps, he or she should understand them immediately without a second thought. "Awaiting reply" achieves that goal, whereas a more flowery or clever single-word label may not.
"Holding" is a possibility. Though, as @phenry explained, the intent of this word could be misunderstood by your readers since it is somewhat ambiguous without further clarification of "holding for a reply."

From Merriam-Webster:

Holding:

a: to delay temporarily the handling of 

"Please hold all my calls."

b : to continue in the same way or to the same degree 

"I hope the weather will hold." —often used with up

Here, holding would imply that you are maintaining a state of expectancy or holding off on any further action until a response is received.