I tested $comcall's callback alias and it's doing the same thing $urlget had been doing. I used the example at

... where i put the /runvbs and cbthread aliases in a remote script, then edited the display to contain $script. I then copied cbthread to aliases.ini and removed the 'alias' keyword.

As described at that page, this launches the callback alias, but it launches cbthread in aliases.ini instead of the alias right next to /runvbs in the same remote script.

//var %t $ticks | runvbs 1 | echo -a $calc($ticks - %t)

So far I've tested 'filter -k' and 'play -a' and they're both calling the alias in the same remote script as they should.