Writing a command that assumes control of the keyboard, like isearch does

[Sean McAfee]

I want to write a command that assumes control of the keyboard until it
returns, highlighting and modifying the current buffer in the meantime,
like the isearch family of commands do.

Specifically, I have some text that is the output from an OCR process,
in which the spaces between words have sometimes been lost; for example:

  GNUEmacsisanextensible,customizabletexteditor—andmore.

I want to write a command that will wait for me to type a letter, then
advance the cursor to that letter and provisionally insert a space after
it, highlighting the passed-over text (the provisional word, that is).
If I then type a space, I confirm the provisional word; if I type
another letter, the provisional space is removed, and the cursor skips
to the newer letter I typed and inserts another provisional space.
Therefore, I could break the words in the sample text above with this
key sequence:

  u SPC s SPC s SPC n SPC e e SPC e SPC t t SPC r SPC d SPC

Some kind of special handling would probably be needed for punctuation,
but I can resolve those details later.  And I suppose RET would probably
terminate the command.

Since the behavior I want to implement broadly resembles what isearch
does, I went and read the source code, but found it unsuitable for
casual reading.  I did note this command near the beginning:

;; For programmed use of isearch-mode, e.g. calling (isearch-forward),
;; isearch-mode behaves modally and does not return until the search
;; is completed.  It uses a recursive-edit to behave this way.

So, does that mean it *doesn't* use a recursive edit when called
interactively?  What does it do in that case?  Not very illuminating.

Browsing the manual, it seems that I might be able to write my own mini
event loop using read-char, but I can't help but feel that there must be
an easier way.  As best I can express it right now, I want to
temporarily install a keymap where every key that normally calls
self-insert-command calls a routine of my choice instead; SPC and RET
have the special meanings described above; and any other key sequence
exits my command and has its normal effect (again, like isearch).

Any pointers to get me on the right track, or even just references to
the appropriate places in the manual that I might have missed, would be
greatly appreciated.