Imodern1878:
Instead of letting the operator check-mark which installed instances of my app should be updated, I decided to give them three choices when it comes to applying an update:
1) Update a single instance of the app
2) Update all instances owned by one customer
3) Update all instances across all customers.

I ended up using a Listbox of customers and a Listbox of application instances owned by that customer.
Data for the listboxes are stored in separate text files. I use the SB "Handle Text File Operation", "Create Text File" and "Set Access Control" to manage the content of the files.