You can set AF W/ Shutter to "Off", AEL w/ Shutter to "off" and that will decouple the settings from the shutter. You can then set one of the custom buttons (c1-c4, center button, four-way buttons) to the function you need. AF-On and AEL Hold/Toggle are available as functions. AF-On behaves exactly like it does on a Nikon. You can then set another button to AEL Hold or Toggle.
AF-On basically acts just like a shutter half-press. If you are in Continuous, it will focus continuously until you let go, where it will hold on to the focus.