Other methods than spanning tree to block loops in F5?

I'm afraid not. It's obviously in F5's interests to be open and support as many standards as possible to ensure their devices can integrate well in most environments, however, as technology such as your example is proprietary it's not really feasible. You wouldn't expect the F5 to be too central to the layer two infrastructure anyway and there are many ways to design and implement them to keep them 'out of the loop'.

In my experience anyway rapid is just that and tends to be faster to converge than the timers might suggest.