I've been wanting to perform this operation, but didn't see the user
interface for it (it could still be there, somewhere!)

So, unless somebody shows me where that is, you can use:
http://fce.cc.gatech.edu/~bolot/squeak/FindAndReplace.17Sep222pm.cs

Based entirely on existing functionality, with very few minor additions.

Choose 'Find and replace' from a menu, you'll first be prompted for the
string to be found (by default, the current search string), and then
prompted for the string to use as a replacement. In MVC, if you hold
left shift when accepting the latter, the change will be made to the
text in the entire view.

alt-g/cmd-g behaves as expected... it repeats the last find or replace.

bolot