Yes, you can easily read/write the SLC from the CLX.
The SLC is older tech than the CLX and doesn't understand the CLX memory format (plus it's just more complicated period)....BUT you can configure a memory area in the CLX to emulate SLC/PLC memory and read/write to it from the SLC.
Since CLX's can read and write to SLC's, and can also emulate them when the CLX is in the slave role, you should be able to do both at the same time and talk between two CLX's in SLC mode, but I've never been able to make it work. You're stuck with produce/consume, but you can't add those at runtime like you can MSG's (SLC/ABETH mode).
Certainly comms between the SLC 5/05 and the Contrologix are straight forward Ethernet messaging. Why the interest in DH+?
Perhaps the SLC is a 5/04 instead of a 5/05? In that case a 1756-DHRIO in the Contrologix backplane will do the interface for you. We've had one running for about 10 years now...works well.