A short question in comp.lang.tcl got me several useful answers that I'm now putting here for later reference:

The (generalized) question:
   * How do I embed comments into an expr (or the first argument to if,while)
The specific context:
   * an expression was structured as an "&&"ed sequence of separate conditions, 
       each of which should be associated with a simple text (e.g. someone's name). 
   * It was considered a valid assumption, that the comment text does not contain "nasty characters".
   * Readability weighs more than performance.

The answers:

by Roy Terry:
   * (slightly modified)
   proc ? {cond args} {return $cond}
   if { [? {<cond1>}  John Doe ]
       && [? {<cond2>}  Fu   Bar ]
       ...
   } ...

   * use Package "Tmac", which allows embedding comments everywhere: [http://www.tclbuzz.com/v0/tmac]
   if { cond1 (* John Doe *)
      && cond2 (*  Fu   Bar *)
      ...
   } ...

by Ulrich Schoebel
   * build up the expression condition by condition:
   set cond [list]
   lappend cond $cond1 ;# John Doe                                                 
   lappend cond $cond2 ;# Fu Bar                                                   
   ...
   if {[expr [join $cond &&]]} { ...                                                                             


Thanks for all the hints!