The visitors took an early lead in the promotion play-off, Harry Morley crossing before Ben Young added the extras.
Moseley, though led 14-7 on 20 minutes as Harry Read slotted twice after scores from Ben Pogson and Austin Nevitt.
Tevita Vaenuku and Birmingham’s Morgan Dawes exchanged unconverted tries before the interval.
A Young penalty cut the gap to four, but a scoreless final 28 minutes leaves Moseley at Level Three for next term.
The final team in the competition will either be Blackheath or London Scottish.
Richmond too strong for Exiles
The latter’s fate was set-up following a 34-11 defeat to Richmond in the 12th-versus-13th Champ play-off game.
Callum Grieve converted a Luc Jones try before Harry Hocking crossed to make it 12-3 to the hosts at half-time, Josh Bellamy with the penalty.
He landed a second to make it a one-score game early in the second period.
But Hocking doubled up, Grieve on target, before Paul Altier went over for 24-6.
A Grieve penalty gave ‘Mond further breathing room before Theo Povey crossed for Scottish.
The home team, however, had the final word thanks to a converted Chidera Obonna effort.
Clud seek to knock off Scottish
National One runners-up Blackheath previously reached the decider with a 22-10 home triumph against Plymouth Albion.
A fourth-minute Iwan Jenkins nudged the visitors in front before Club replied with an Archie Smeaton try.
Sam Morley converted for 7-3 at half-time, and Finn Osborne then crossed five minutes into the second period.
A Tom Hughes seven-pointer came soon after before Morley slotted a penalty.
And while Jenkins landed after Harry Legg’s try late in the third quarter, Blackheath held on comfortably.
