Is there anyway to create a one-way replica of a FGDB to ArcSDE. I want the FGDB to be editable, and the ArcSDE replica read-only.
No, the scenario you mention is not possible.
To create a replica, the Parent gdb has to be SDE.
In 1-way Parent to Child: Parent is ArcSDE and Child can be File\sde gdb. But the editing will be in the Parent sde gdb.
In 1-way Child to Parent: Both the Parent and Child have to be an sde gdb.
In 2-way replica: Both have to be an sde gdb.
So your only option remains, creating a Checkout Replica....but in this scenario, you will have to recreate replica after every synchronization process.
I was afraid of that. This is even if you make a GeoData Service out of the FGDB?
Yes, I believe so. However I'll let one of the Server Experts on the forum to confirm that and suggest any other solution if possible.
I'm not an expert, but was playing around with replicas and check-outs a couple months ago and here are a few links that I found helpful
Retrieving data ...