I looked in mod_openzap.c and I didn't see any references to channel
variables. However, you have context and dialplan options. I suggest that
you create a dialplan context just for your FXS port(s). Try this. Create
conf/dialplan/fxs-ports.xml:

<include>
  <context name="fxs-ports">
    <extension name="outbound">
      <condition field="destination_number" expression="^(.*)$">
        <action application="set"
data="toll_allow=local,domestic,international"/>
        <action application="transfer" data="$1 XML default"/>
      </condition>
    </extension>
  </context>
</include>

Then in your openzap.conf.xml change the context for the analog span(s) with
the FXS ports:
<param name="context" value="default"/>

Restart FS after making these changes and then give it a shot. You should
see the call from the analog phone going into context "fxs-ports" and then
get transferred over to the default context where it will act like your SIP
phones because we manually set the ${toll_allow} chan var.

-MC