Noting here for posterity that QLab 4.6.6 includes a fix that re-enables the ability to script the background color of a Text cue. The correct name of the AppleScript property to use going forward is "backgroundRgbaColor".
Below is an example script that will set the background color of cue "1" to RGBA 20% 10% 40% 100%.
tell application id "com.figure53.QLab.4" to tell front workspace
set theBackgroundColor to {red:0.2, green:0.1, blue:0.4, alpha:1.0}
set theCue to cue "1"
set theFormat to text format of theCue
set theNewFormat to {}
repeat with substringFormat in theFormat
try
set backgroundRgbaColor of substringFormat to theBackgroundColor # attempt to overwrite an existing color record, if one exists
on error
set substringFormat to (substringFormat & {backgroundRgbaColor:theBackgroundColor}) # else add a new color record to the current format
end try
copy substringFormat to the end of theNewFormat
end repeat
set text format of theCue to theNewFormat
end tell