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: 
How about "Unanswered"?

I know it's not the perfect answer -- it's not clear whether the items are unanswered by other people or unanswered by you. But then, the same could be said of "awaiting reply" :-)
"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.