the scenario that you described is just ideal for two different contexts. Why do you want to have them in one?

Of course you can set a variable in an "inline" action in the beginning of a context, like this:

<action application="set" data="call_direction=inbound" inline="true"/>

and later in the dialplan, use this variable in conditions.
Also the variable can be assigned at the SIP gateway, then incoming calls will have it automatically:

<gateway name="gateway012">
   <param name="realm" value="sip.voipcarrier.com" />
   <param name="username" value="WBrandes" />
   <param name="password" value="myvoiceismypassword" />
   <param name="register" value="true" />
   <param name="caller-id-in-from" value="true" />
   <param name="ping" value="5" />
   <param name="ping-max" value="3" />
   <param name="retry-seconds" value="5" />
   <param name="expire-seconds" value="60" />
   <variables>
     <variable name="this_is_inbound_call" value="true" direction="inbound"/>
   </variables>
 </gateway>

direction="inbound" means to apply this variable for inbound calls.