How to execute a tcl script in an existing HM session whose PID is known

Yes, your hm session must work as a server. Use dde or socket command

Refer above.

In your case, this way may be simpler:

Your hm session (eg. with pid 1234) must pre-run a timer (write it in c:/users/name/hmcustom.tcl)

That timer checks a file eg. 1234.tcl

If file existing, source it, then clear the file!

So everytime you want to excute a script, put it in 1234.tcl

A such timer:

proc ::p_MonitorFile {{path {}}} {

   set path_ $path

   after cancel [list ::p_MonitorFile $path_]

   if {$path==''} {

      set path [file join $::env(HOME) [pid].tcl]

   }

   if {[file exists $path]} {

       catch {source $path} err

       file delete $path

       puts 'Executed $path got $err'

   }

   after 3000 [list ::p_MonitorFile $path_]

}