Hmm.  Let's set the ball rolling...

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A simple way to get a read-only global variable is to use a suitable trace:

     set ROglobal someValue
     trace variable ROglobal w "[list set ::ROglobal $ROglobal];error read-only;#"

Notice the time- and hair-saving tricks:
   1. Using [[list]] to construct a ''guaranteed'' safe command for later execution.
   1. Using colon notation to force a reference to a global variable, whatever the context.
   1. Inserting the global name of the variable in the trace command, instead of working with its local referent.
   1. Using a trailing ";#" to trim the undesirable extra arguments from the trace command.

'''DKF'''
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Neat...  I remember having had trouble when the trace was set on '''::var''', yet usage was based on
'''global var''', or the other way around.
Does anyone know what the exact behavior is when mixing these two approaches?

-- JC