It may be a workaround, but you can always pass a special character for $null (this if for if you aren't using an alias, though it could be set up for use with the alias as well)...
Pick a character (such as ~):
if (%A == $null) var %A = ~
Or, use the word NULL:
if (%A == $null) var %A = NULL
Then, you can just ignore that character (or word) when looking at the data... it's basically just a spacer.
Another option would be to put leading and/or trailing characters on all variables (this is for either with or without the alias)...
var %A = .1.
Then, when you get a $null, you have .. with nothing else (easy to notice). And the leading/trailing .'s are easy to remove when getting the data. You could, of course, choose another character instead of .'s. It was just an example.
I think that changing $null would be very difficult as I believe all commands will ignore $null as if it's not there. To change it for this would mean it would need to be changed everywhere.
