Unless there's one I can't find, it would be great to have this. I use it all the time in Max - you just tap spacebar and it locks your current selection so you can do all you want to it, but you cant select anything else.
And I think the script for "LockUnselected" under there will do what you want.
Copy the command part - that's the part on the right that starts with script: - and then inside of MoI go to Options / Shortcut keys, and add a new entry and paste that script into the right-side column.
For the left-side column, enter the key combo that you want to trigger it, you can type in Space there if you want the space bar to trigger that.