Nevermind - It's been a while since I've gotten into the weeds with RM4 and I had a minor brain fart. I was able to use the "variable connector" device attribute ("variable") in an RM5.1 rule to make the magic happen (i.e., detect when the RM4 Variable changed).
Note that when I tried to add a "*changed*" trigger ("When Variable Reports MyTimeVar Changed") to an RM4 rule, that blew the rule up. Whenever I select "changed" from the drop down, it blows up with:
Saying "changed" is an invalid time.
Since RM4 isn't supported anymore, I used the "variable" attribute for the associated Variable Connector Device. I just had to blow the cobwebs off on RM4 behavior. 
