RM vs IM is entirely about where the name (id, pointer, whatever) of the widgets comes from. In RM it comes from the library, and is typically a pointer to some data that represents the 'widget', and in IM it comes from the user and is frequently a string or other local pointer (sometimes macros are used to generate IDs).

Anyway, there's no reason this would be harder to do in IM, other than that a lot of people are more familiar implementing RM than IM.