@mod This is a great solution, thank you. Here is modified to what I needed it to do:

expr.send.zip

It would've been nice to have been able to change the expression by sending a message but as it turns out it's very complicated indeed for what it is, and this will do nicely.

Thanks everyone for your help.

@weightless HOT!

I have updated the patch with a help file, now the last states of a specific object are accessible by addressing them in chronological order (0 = last state, 1 second to last etc.)

expr.cell.zip

I'm six years late to the conversation, but I'm adding my own contribution because this is the page that I found in the search engine when looking for "setting [expr] dynamically". Perhaps others will find it here.

calc.zip