I tried to add a new task to my task graph, and that task used sstate.  The task was called do_log_srcrev, and I added the following to my bbclass to try to enable sstate:

SSTATETASKS += "do_log_srcrev"
do_log_srcrev[sstate-name] = "log-srcrev"
do_log_srcrev[sstate-inputdirs] = "${SRCREV_WORKDIR}"
do_log_srcrev[sstate-outputdirs] = "${SRCREV_INSTALL_DIR}"

python do_log_srcrev_setscene () {
        sstate_setscene(d)
}
addtask do_log_srcrev_setscene

After I inherited the class and built, I hit a nasty error that ended like this:

 File "<string>", line 1, in <module>
  File "sstate.bbclass", line 16, in sstate_checkhashes
<omitted mounds of stuff passed to the function>
KeyError: 'do_log_srcrev'

I looked over in sstate.bbclass at the top of the sstate_checkhashes() function to find the source of my problem:

       # This needs to go away, FIXME
    mapping = {
        "do_populate_sysroot" : "populate-sysroot",
        "do_populate_lic" : "populate-lic",
        "do_package_write_ipk" : "deploy-ipk",
        "do_package_write_deb" : "deploy-deb",
        "do_package_write_rpm" : "deploy-rpm",
        "do_package" : "package",
        "do_deploy" : "deploy",
    }

    for task in range(len(sq_fn)):
        sstatefile = d.expand("${SSTATE_DIR}/" + sq_hashfn[task] + "_" + mapping[sq_task[task]] + ".tgz")

If I added a mapping for my task, the problem disappeared and everything worked normally.  However, I figure it shouldn't be necessary to modify the base tools in order to make my task use sstate.  If you git show 35d095c2, you should be able to see how this was added.  Can it be made to go away by using information like the sstate-name?  Without a fix for this, it would seem I'm in a situation where I can't write custom tasks without maintaining local patches for this, which is something I'm trying to avoid.