Unfortunately I could never get any of Kyle's suggestions to work, including the use of the API triggers.
Here's the solution I came up with...
On line 3070 of comprofiler.html.php, I added the following NEW trigger after the registration template builds:
[code:1]$results = $_PLUGINS->trigger( 'onAfterRegisterFormDisplay', array( &$user, $regErrorMSG ) );
[/code:1]
Then, I followed Kyle's Tutorial on building a plug-in and calling a trigger.
Within my trigger function, I add a Javascript function that sets the form field default value:
[code:1]$cb_fname = JRequest::getVar('mem_user_fname');
echo "<script type = \"text/javascript\">";
echo "document.adminForm.firstname.value = \"$cb_fname\";";
echo "</script>";
[/code:1]
I ended up hardcoding a lot of stuff, including the name of the incoming values (access through a POST var) and the name of the form fields (document.adminForm.firstname.value). But that still better than not having anything at all.
Hope this helps someone else!