Just for kicks, tried this:

oo=createobject("custom")
addproperty(oo, "arr[3,2]","zz")
oo.arr[1,1]="bbzz"
?getpem(oo, "arr")
?getpem(oo, "arr[2,2]")

The debugger shows that oo has an array property, all six elements of which have the value of "zz" except "bbzz" in the first. The first output shows a "zz", the second throws an error, "Function argument value, type, or count is invalid.", which means that the 2nd parameter will accept only names, not names with index. Ditto when trying with a single index.

Too bad, I just wanted to avoid using a bunch of macros to store about a dozen arrays into properties of an object. Now it will have to be the same old quick & dirty.