Is it possible to pass hidden variables with the CFGrid and do a CFGridUpdate
using that hidden variable? Or even better yet, is it possible to default a
value in a CFGrid cell. I'm using the Flash format, not applet. Seems like
this should be a built-in thing but I can't find it in the help. Thanks!

I have not been able to set defaults on a CFGrid using Flash format but I have
been able to manually default the field value.

Try something like this.
<cfsavecontent variable="Add_clicked">
GridData.insertRow(UserService);

UserService.dataProvider.editField(UserService.selectedIndex,'OC_PAYROLL_NUMBER'
,PayNo.text);
UserService.dataProvider.editField(UserService.selectedIndex,'CURRENT','Y');
</cfsavecontent>

The save content is called by the onclick value of the insert button eg
<cfinfput type="button" name="Insert" label="Insert" onclick="#Add_clicked#">

With regard to Hidden fields - I have not had much success manipulating them
with action script but have found that using a cfinput of type="text" with
visible ="no" can be manipulated using action script using code like
Statuscheck.text = 'help';
where Statuscheck is the name of the cfinput

Beware - action script is CASE SENSITIVE if a function doesn't seem to work
right check the case

Hope this helps