Is this a workable water softener plan?
Yes, You have neglected to show how any particular sized water softener would fit onto a shelf table size of 15" x 20"; let alone get it into that space. 

Also you will need a riser on the softener side of the P-Trap that you have shown there. Seems to me that there will also need to be a guaranteed airgap between softener drain line and P-Trap riser. If this were practical you may want to consider placing the P-Trap fitting below the cleanout or combine it in with a replacement for that fitting.

Lastly it seems like it will be a major pain to get access to the softener unit when elevated like that to service it. Service can include having to add softener salt to the tank for some units. For salt-free units that have 3-5 year replaceable resin type canisters the overall height of the unit may be a problem.