Seeking RGB Chase Pro Tip

[Bizywk]

When I was more of a novice, I just plowed through a number of issues manually which was slow and inefficient, but worked.   Now that I understand the tools and features much better, I'm looking for ways to program smarter and faster. 
RGB Chases have been troublesome for me from the beginning.  They really work great if you're working with a blank canvas. 
 
The entire square RGB chase space beginning to end of where you want the diagonal chase area to be has to be completely blank before programming the chase or all you get is a vertical copy (All pixels simultaneously on then off at the same time).   I've tried a variety of combinations of chase and foreground/background tests, but they don't give me the desired effect. At best, I just get the vertical copy and paste which is similar to if I just cut and pasted vertically a number of times
 
What I want is to be able to highlight a few boxes, select the chase tool, drag through the desired diagonal chase space and if the desired chase box already has programming such as a single color already there, that it simply lays the diagonal chase over top.

 

Is there a method out there that will do that?