I have my D-700 configured with settings from the referenced page and Field Day call... and stored in a PM (program memory). But, who do I send a message to?

Chicken-egg problem I guess... SO everyone should send their messages to ALL so that not only can they originate a message, but everyone else with a D7 or D700 can receive it... And thereby know who they can QSO!

Are position beacons from the 145.93 uplink also being passed as well as the messages from 145.85 to the downlink so it's a matter of looking at the station list and sending a message to anyone you see as in normal APRS terrestrial ops?

Yes and Good points!. GO-32 is unique when operating with the D700, D7 etc radios. The POSITION formats only go through on the 145.93 uplink, but messages only go through on the 145.85 uplink. SO I guess it best to have both channels in memory. Beacon your posit a few times on .93 and watch for other stations. Send all your messages to ALL and include any other callsigns in the text of the QSL message.

BE SURE TO CLEAR OUT YOUR MESSAGE memories before a pass so that you have room to capture any incoming. If your memories are full, your radio will reject any new messages.

How to use GO32 for APRS: http://eng.usna.navy.mil/~bruninga/GO32-ops.html