In Coliseum with my Rear Admiral Lower Half, the available melee weapons are all for Rear Admiral Upper Half, and therefore unequippable. Fortunately I had crafted one in Memory Alpha, or I would not have been able to advance.
i had the same problem, the weapons available were RA upper, but i was a captain 8, had to do the whole thing hand to hand. took a while and several resporns. however the blue lurpa you can pick up just before you leave the arena was lvled correctly. too little to late.