I'm trying to find a reference in newer documentation, but at least in version 10.2 it would appear that oneconnect MIGHT impact the SNAT.
From: http://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/ltm_configuration_guide_10_1/ltm_snat.html
"The BIG-IP system OneConnectTM feature allows client requests to re-use idle server-side connections. Without a SNAT, the source IP address in the server-side connection remains the address of the client node that initially established the connection, regardless of which other client nodes re-use the connection. Although this is not an issue for traffic routing, you might find it confusing when examining various types of system output. A SNAT solves this problem."
This would lead me to believe that even with SNAT, it will use the SNAT of the first connection, and not re-evaluate it for each connection.
Might be best to ask support at this point. I use a rule exactly like your example without oneconnect and it works fine.