Populating a formselect with plugin output

We have created a plugin that is basically a modified version of the
sessions plugin that always return a list of all users currently logged into
the wiki. We'd like to display those values in a FormSelect drop down box.
We're currently trying to set a variable from the output of the plugin and
then use that variable as the input to the FormSelect.

[{SET allusers=[{PluginTest}]}]
[{FormSet form='testForm'}]
[{FormOutput form='testForm' handler='myHandler' populate='handler'}]
[{FormOpen form='testform'}]
[{FormSelect name='userlist' value='{$allusers}'}]

The last line there results in a drop-down box containing {PluginTest} as
its only value. It seems that PluginTest is not being evaluated in line 1,
and thus allusers is being set to the plugin name rather than the list of
users. We've tried several variations of syntax without success. Any help
you could provide would be great.

Thanks,
Bill Tollett