Hi Ludovic,
Sadly, I nowadays only have time to look at guile-devel briefly now and then. I did this now and happened to see this.
Just wanted to say that I think that we (or at least I) at some point in time had the goal to replace structs with pure GOOPS data structures. In the context of FFI, this would allow you to be more flexible than what structs allow, ultimately being able to access arbitrary C structs and C++ structs/classes directly from Scheme. I find that a proper MOP (which maybe still is not fully developed) is a nicer way to handle non-standard access than the strange character strings in struct vtables...