I recently created a new job and I thought tied it to a particular slave. Later
I realized that I had in fact forgotten to tie it to that slave, and the first
few builds ran on master. I corrected this and now it is running on the slave as
desired.

However now the master has an old copy of the job's workspace too. Since the
workspace is ~1 Gb, I would like to clean it up. But I want to leave the
actively used workspace on the slave intact, since recreating it is much slower
than running an incremental build - needs to do a full SCM checkout and then do
some big downloads.

Unfortunately there seems to be no way through Hudson's GUI to delete the
workspace on a particular node (master, in my case). I presume "Wipe Out
Workspace" would delete all copies.

Not sure what the best GUI would be, but perhaps the .../wipeOutWorkspace page
which displays a confirmation button could have a list of checkboxes, all
initially checked, listing the nodes on which a copy of the workspace currently
resides (if there is >1 such node). You could uncheck some of them if you
wished. The node on which the last build ran should be highlighted.