Hi Stefan!
For me, personally, I would be glad to sign a general assignment. Unfortunately, my employer does not want to sign a general sign-off agreement -- they prefer to do this case by case. (I respect this -- after all, the company I work for develop IDE:s for the embedded industry so there is a risk of conflict of interests, even though I mainly work on the compiler and not the editor.)
In this case, for isearch.el, I do not see the need for an assignment, as I only have reported bugs I have found, not contributed actual code. However, if you think it is necessary, I will sign an assignment -- hopefully we can define a scope that is acceptable to my employer, like "basic Emacs functionality".
-- Anders