Here is a patch that implements support for transient input methods
in Isearch mode.

While 'C-x 8' simply adds own keys to the default isearch-mode-map,
input methods have to rely on isearch-process-search-multibyte-characters
in lisp/international/isearch-x.el.  Since it uses 'read-string'
to read a character, currently the activated transient input method
is deactivated in the minibuffer of 'read-string' that is a wrong place,
so needed to introduce a new separate function deactivate-transient-input-method,
thus renamed the previous function to activate-transient-input-method,
and had to add more global variables.  But in result everything works fine: