I don't know if in the lab would be allowed but in this task i have bound a static default route to null0 interface on R5.
That caused the advertising of an internal EIGRP default route without the use of the summary.
This config won't cause the hide of the interfaces connecting the backbones but will cause the hide of the external prefixes.
We will need anyway to redistribute from BGP the AS 254 prefixes to avoid the routing loop via the default route.