I think the added value of having this feature does not outweigh the considerable effort to implement it. The Springsource-led OSGi hype has sort of quieted down, probably because people have found out it is a Really Hard Thing to do, especially when using software like Hibernate.
Only in very specialized usecases does the idea of 'look i can replace a bundle' really have benefit.
Furthermore, Wolfgang's OSGI plugin already allows a grails app to run in an OSGi container (albeit as a monolith), for those that really have to.
I would vote for resolving this with a 'won't fix'...