When LSQL calls the plugin have it check the value of Which_Region and if there is no value push the default region name into it, i.e.; nobanner
Then it will either display a banner (found a value in Which_Region), or else the default nobanner region would be called and display the 1x1 pixel banner.
I'm also assuming you'd need to change the plugin a little to 'check' and insert the default name. Maybe something along the lines of:
Does this make any sense?
I realise that you'd really wanted a method of not using the plugin if no region was assigned, but this way you'd have the advantage of being able to update your 'default' whenever you wanted, maybe you might decide to run some PPC ads in the default spaces later and with this solution you'd just create a new banner to run in the nobanner region
All the best